The Company recognized stock-based compensation expense during the three and nine months ended September 30, 2011 of $12.1 million and $36.2 million, respectively, and stock-based compensation expense during the three and nine months ended September 30, 2010 of approximately $13.4 million and $40.1 million, respectively. Stock-based compensation expense for the nine months ended September 30, 2011 includes $3.0 million related to the modification of the vesting and exercise terms for certain employees' equity awards. The Company capitalized $2.4 million of stock-based compensation expense as property and equipment during the three and nine months ended September 30, 2011 and did not capitalize any stock-based compensation expense for the three and nine months ended September 30, 2010.

Stock Options—The following table summarizes the Company's option activity for the nine months ended September 30, 2011:


		Number of Options

Outstanding as of January 1, 2011		7,962,566

Granted		1,114,584
Exercised		(1,827,258)
Forfeited		(178,149)
Expired		(3,150)

Outstanding as of September 30, 2011		7,068,593

The Company estimates the fair value of each option grant on the date of grant using the Black-Scholes pricing model. The following assumptions were used to determine the grant date fair value for options granted during the nine months ended September 30, 2011:


Range of risk-free interest rate	0.90% - 2.24%
Weighted average risk-free interest rate	1.99%
Expected life of option grants	4.5 years
Range of expected volatility of underlying stock price	36.89% - 37.94%
Weighted average expected volatility of underlying stock price	36.96%
Expected annual dividends	N/A

The weighted average grant date fair value per share during the nine months ended September 30, 2011 was $17.19. As of September 30, 2011, total unrecognized compensation expense related to unvested stock options was $30.0 million, and is expected to be recognized over a weighted average period of approximately two years.

Restricted Stock Units—The following table summarizes the Company's restricted stock unit activity during the nine months ended September 30, 2011:


		Number of Units

Outstanding as of January 1, 2011		2,200,225
Granted		899,259
Vested		(717,477)
Forfeited		(129,842)

Outstanding as of September 30, 2011		2,252,165

As of September 30, 2011, total unrecognized compensation expense related to unvested restricted stock units was $71.4 million, and is expected to be recognized over a weighted average period of approximately two years.

Employee Stock Purchase Plan—The Company maintains an employee stock purchase plan (“ESPP”) for all eligible employees as described in note 12 to the Company's Annual Report on Form 10-K for the year ended December 31, 2010. The offering periods run from June 1 through November 30 and from December 1 through May 31 of each year. During the nine months ended September 30, 2011, employees purchased 43,485 shares under the ESPP and the fair value of such shares was $11.88.

Key assumptions used to apply the Black-Scholes pricing model for shares purchased through the ESPP during the nine months ended September 30, 2011 are as follows:


	Approximate risk-free interest rate		0.20%
	Expected life of shares		6 months
	Expected volatility of underlying stock price		34.55%
	Expected annual dividends		N/A