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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-02-14</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001509441</rptOwnerCik>
            <rptOwnerName>Schroepfer Michael Todd</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>
            <isOfficer>1</isOfficer>
            <officerTitle>Chief Technology Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
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            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-02-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>C</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
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                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
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                    <value>557268</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
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            </transactionDate>
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                <footnoteId id="F2"/>
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            <transactionTimeliness></transactionTimeliness>
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                </transactionShares>
                <transactionPricePerShare>
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                </transactionPricePerShare>
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            </transactionDate>
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                <footnoteId id="F2"/>
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        <nonDerivativeTransaction>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
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            </postTransactionAmounts>
            <ownershipNature>
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        <nonDerivativeTransaction>
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            </transactionDate>
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            <transactionTimeliness></transactionTimeliness>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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        </nonDerivativeTransaction>
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            <securityTitle>
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            <transactionTimeliness></transactionTimeliness>
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                    <footnoteId id="F5"/>
                </transactionShares>
                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>592831</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-02-15</value>
            </transactionDate>
            <transactionCoding>
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                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>17806</value>
                    <footnoteId id="F6"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>133.85</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>575025</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-02-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
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                </transactionShares>
                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>585048</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
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        <nonDerivativeTransaction>
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        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy Class B Common Stock)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>1.854</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2017-02-14</value>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-01-11</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F8"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>66256</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
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            <ownershipNature>
                <directOrIndirectOwnership>
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        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
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            <conversionOrExercisePrice>
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            <transactionDate>
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                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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                <transactionShares>
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                </transactionShares>
                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <exerciseDate>
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            <expirationDate>
                <footnoteId id="F8"/>
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            <underlyingSecurity>
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            </underlyingSecurity>
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        <derivativeTransaction>
            <securityTitle>
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            <transactionDate>
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            <transactionAmounts>
                <transactionShares>
                    <value>34123</value>
                    <footnoteId id="F9"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F8"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F8"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>34123</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</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="F13"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2017-02-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>10023</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F14"/>
            </exerciseDate>
            <expirationDate>
                <value>2024-03-16</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10023</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>120270</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 options listed in Table II.</footnote>
        <footnote id="F2">The sales reported were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person.</footnote>
        <footnote id="F3">The reported price in Column 4 is a weighted average price.  These shares were sold in multiple transactions at prices ranging from $132.57 to $133.56 per share, inclusive.  The reporting person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</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 $133.57 to $134.01 per share, inclusive.  The reporting person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon 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">Represents the number of shares that were acquired upon conversion of Class B Common Stock to Class A Common Stock in connection with the settlement of the Restricted Stock Units (&quot;RSUs&quot;) listed in Table II.</footnote>
        <footnote id="F6">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="F7">The option vested as to 1/5th of the total shares on October 29, 2009, after which 1/60th of the total shares vest monthly, subject to continued service through each vesting date. In connection with certain estate planning transfers, the reporting person transferred vested shares underlying the option to Michael Schroepfer and Erin Hoffman, Co-Trustees of The HS Trust u/a/d 9/28/11.</footnote>
        <footnote id="F8">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="F9">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="F10">Each RSU represents a contingent right to receive 1 share of the issuer's Class B Common Stock upon settlement.</footnote>
        <footnote id="F11">The RSUs were granted with both (a) a liquidity event-based vesting condition and (b) a service-based vesting condition, both of which conditions must be satisfied in order for the RSUs to vest.  The liquidity event-based vesting condition was satisfied on November 17, 2012.  The service-based vesting condition was satisfied as to 1/16th of the total number of shares on August 15, 2014, after which 1/16th of the total number of shares vest quarterly, subject to continued service through each vesting date.</footnote>
        <footnote id="F12">The RSUs vest as to 1/16th of the total shares quarterly, beginning on November 15, 2014, subject to continued service through each vesting date.</footnote>
        <footnote id="F13">Each RSU represents a contingent right to receive 1 share of the issuer's Class A Common Stock upon settlement.</footnote>
        <footnote id="F14">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>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Michael Johnson as attorney-in-fact for Michael T. Schroepfer</signatureName>
        <signatureDate>2017-02-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
