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Stockholders' Equity (Deficit)
 3 Months Ended
Mar. 31, 2015
Equity and Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stockholders' Equity (Deficit)
Stockholders’ Equity
Common Stock
As of March 31, 2015, we had 58,442,009 shares of Class A common stock and 34,628,791 shares of Class B common stock outstanding.
Stock Options Activity
A summary of stock option activity for the three months ended March 31, 2015 is as follows (in thousands, except share and per share amounts): 
 
Options
Outstanding
 
Weighted-
Average
Exercise
Price
 
Aggregate
Intrinsic
Value
Balance at December 31, 2014
16,392,539

 
$
4.40

 
$
128,541

Stock option grants
466,900

 
$
8.88

 
 
Stock options exercised
(1,925,731
)
 
$
1.02

 
 
Stock options canceled
(1,016,745
)
 
$
7.12

 
 
Balance at March 31, 2015
13,916,963

 
$
4.82

 
$
61,110


The total grant-date fair value of stock options granted during the three months ended March 31, 2015 and 2014 was $2.1 million and $25.9 million, respectively.
The fair value of each option grant was estimated on the date of grant using the Black-Scholes option-valuation model with the following assumptions and fair value per share:
 
Three Months Ended March 31,
 
2015
 
2014
Volatility
53%
 
60%
Expected life (in years)
6.2
 
5.0-6.2
Risk-free interest rate
1.38%-1.79%
 
1.53%-1.95%
Dividend yield
—%
 
—%

As of March 31, 2015, we had $32.6 million in unrecognized compensation cost related to non-vested stock options, which is expected to be recognized over a weighted-average period of approximately 3.0 years.
Restricted Stock Units

A summary of restricted stock unit activity for the three months ended March 31, 2015 is as follows (in thousands, except share and per share amounts): 
 
Number of
Shares
Outstanding
 
Weighted-
Average
Grant Date Fair Value
Balance at December 31, 2014
1,398,893

 
$
11.06

Restricted Stock Units granted
666,300

 
$
9.13

Restricted Stock Units vested
—

 
$
—

Restricted Stock Units forfeited
(131,600
)
 
$
10.40

Balance at March 31, 2015
1,933,593

 
$
10.44

As of March 31, 2015, there was a total of $20.2 million in unrecognized compensation cost related to restricted stock units, which is expected to be recognized over a weighted-average period of approximately 3.9 years.