<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-08-14</periodOfReport>

    <issuer>
        <issuerCik>0001326801</issuerCik>
        <issuerName>Facebook Inc</issuerName>
        <issuerTradingSymbol>FB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001366010</rptOwnerCik>
            <rptOwnerName>Sandberg Sheryl</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O FACEBOOK, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>1601 WILLOW ROAD</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
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            <isOfficer>1</isOfficer>
            <officerTitle>Chief Operating Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

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                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F13"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F13"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>55000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>55000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Sheryl K. Sandberg, Trustee of Sheryl K. Sandberg Revocable Trust UTA dated September 3, 2004</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Class B Common Stock</value>
                <footnoteId id="F13"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F13"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-08-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>C</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>55000</value>
                    <footnoteId id="F15"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F13"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F13"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>55000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Sheryl K. Sandberg, Trustee of Sheryl K. Sandberg Revocable Trust UTA dated September 3, 2004</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units (RSU) (Class A)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F16"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-08-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>34364</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F17"/>
            </exerciseDate>
            <expirationDate>
                <value>2023-05-05</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>34364</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>309279</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units (RSU) (Class A)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F16"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-08-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>13030</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F18"/>
            </exerciseDate>
            <expirationDate>
                <value>2024-03-16</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>13030</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26059</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units (RSU) (Class A)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F16"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-08-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>16508</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F19"/>
            </exerciseDate>
            <expirationDate>
                <value>2025-03-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>16508</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>82543</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units (RSU) (Class A)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F16"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-08-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>11243</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F20"/>
            </exerciseDate>
            <expirationDate>
                <value>2026-03-14</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>11243</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>168647</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units (RSU) (Class A)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F16"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-08-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>9426</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F21"/>
            </exerciseDate>
            <expirationDate>
                <value>2027-03-14</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>9426</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>122541</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units (RSU) (Class A)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F16"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-08-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>6847</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F22"/>
            </exerciseDate>
            <expirationDate>
                <value>2028-03-19</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6847</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>102719</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units (RSU) (Class A)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F16"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-08-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>7434</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F23"/>
            </exerciseDate>
            <expirationDate>
                <value>2029-03-19</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>7434</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>104081</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents the number of shares that were acquired upon the conversion of Class B Common Stock to Class A Common Stock in connection with the exercise of the stock option listed in Table II.</footnote>
        <footnote id="F2">Shares held of record by Sheryl K. Sandberg, Trustee of Sheryl K. Sandberg Revocable Trust UTA dated September 3, 2004.</footnote>
        <footnote id="F3">The sales reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the holder.</footnote>
        <footnote id="F4">The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $179.36 to $180.355 per share, inclusive. The holder undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F5">The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $180.36 to $181.35 per share, inclusive. The holder undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F6">The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $181.40 to $182.38 per share, inclusive. The holder undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F7">The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $182.44 to $183.43 per share, inclusive. The holder undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F8">The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $183.465 to $184.46 per share, inclusive. The holder undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F9">The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $184.47 to $185.46 per share, inclusive. The holder undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F10">The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $185.48 to $185.99 per share, inclusive. The holder undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F11">Represents the number of shares of Class A Common Stock that have been withheld by the issuer to satisfy its income tax withholding and remittance obligations in connection with the net settlement of the RSUs and does not represent a sale by the reporting person.</footnote>
        <footnote id="F12">The option vests as to (a) 260,000 shares in equal monthly installments over 48 months, beginning on May 1, 2013 and then (b) the remaining 940,000 shares vest in equal monthly installments over the following 12 months, subject to continued service through each vesting date.</footnote>
        <footnote id="F13">The Class B Common Stock is convertible into the issuer's Class A Common Stock on a 1-for-1 basis (a) at the holder's option or (b) upon certain transfers of such shares, and has no expiration date.</footnote>
        <footnote id="F14">Options held of record by Sheryl K. Sandberg, Trustee of Sheryl K. Sandberg Revocable Trust UTA dated September 3, 2004.</footnote>
        <footnote id="F15">The holder elected to convert the shares of Class B Common Stock into Class A Common Stock on a 1-for-1 basis.</footnote>
        <footnote id="F16">Each Restricted Stock Unit (&quot;RSU&quot;) represents a contingent right to receive 1 share of the issuer's Class A Common Stock upon settlement.</footnote>
        <footnote id="F17">The RSUs vest as to 1/16th of the total shares quarterly, beginning on November 15, 2017, subject to continued service through each vesting date.</footnote>
        <footnote id="F18">The RSUs vest as to 1/16th of the total shares quarterly, beginning on May 15, 2016, subject to continued service through each vesting date.</footnote>
        <footnote id="F19">The RSUs vest as to 1/12th of the total shares quarterly, beginning on February 15, 2018, subject to continued service through each vesting date.</footnote>
        <footnote id="F20">The RSUs shall vest quarterly as to 1/16th of the total shares, commencing the first quarter following May 15, 2019, subject to continued service through each vesting date.</footnote>
        <footnote id="F21">The RSUs shall vest quarterly as to 1/16th of the total shares, commencing the first quarter following November 15, 2018, subject to continued service through each vesting date.</footnote>
        <footnote id="F22">The RSUs shall vest quarterly as to 1/16th of the total shares, commencing the first quarter following May 15, 2019, subject to continued service through each vesting date.</footnote>
        <footnote id="F23">The RSUs vest quarterly as to 1/16th of the total RSUs, commencing the first quarter following February 15, 2019, subject to continued service through each vesting date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ David Kling as attorney-in-fact for Sheryl K. Sandberg</signatureName>
        <signatureDate>2019-08-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
