The Company grants stock-based awards, including stock options, restricted stock awards, and restricted stock units. The Company measures stock-based compensation cost at the grant date based on the fair value of the award and recognizes it as expense over the vesting or service period, as applicable, of the stock-based award using the straight-line method. The Company accounts for forfeitures as they occur.

Stock-based compensation expense was included in the following operating expense categories as follows (in thousands):

	Three months ended March 31,
	2018		2019
Service costs	$	128	$	59
Sales and marketing		214		177
Product development		91		76
General and administrative		518		233
Total stock-based compensation	$	951	$	545

The Company uses the Black-Scholes option pricing model to estimate the per share fair value of stock option grants with time-based vesting. The Black-Scholes model relies on a number of key assumptions to calculate estimated fair values. For the three months ended March 31, 2018 and 2019, the expected life of each award granted was determined based on historical experience with similar awards, giving consideration to contractual terms, anticipated exercise patterns, vesting schedules and expirations. Expected volatility is based on historical volatility levels of the Company’s Class B common stock and the expected volatility of companies in similar industries that have similar vesting and contractual terms. The risk-free interest rate is based on the implied yield currently available on U.S. Treasury issues with terms approximately equal to the expected life of the option. For the three months ended March 31, 2018, the Company used an expected dividend yield for those grants prior to the record date of the Company’s common stock cash dividend payment in March 2018.

The following weighted average assumptions were used in determining the fair value of time-vested stock option grants for the periods presented:

	Three months ended March 31,
	2018	2019
Expected life (in years)	4.0	4.0
Risk-free interest rate	2.54%	2.22%
Expected volatility	53%	40%
Expected dividend yield	0% - 3.57%	0%

Stock option activity during the three months ended March 31, 2019 is summarized as follows:

	Shares (in thousands)			Weighted average exercise price		Weighted average remaining contractual term (in years)
Balance at December 31, 2018		5,511		$	4.98		5.53
Options granted		110			4.03
Options forfeited		(160	)		3.23
Options expired		(123	)		5.90
Options exercised		(55	)		3.30
Balance at March 31, 2019		5,283		$	5.01		5.05

Restricted stock awards and restricted stock units are generally measured at fair value on the date of grant based on the number of awards granted and the quoted price of the Company’s common stock. Restricted stock units entitle the holder to receive one share of the Company’s Class B common stock upon satisfaction of certain service conditions.

Restricted stock awards and restricted stock unit activity during the three months ended March 31, 2019 is summarized as follows:

	Shares/ Units (in thousands)			Weighted average grant date fair value
Unvested balance at December 31, 2018		1,264		$	3.28
Granted		120			4.18
Vested		(31	)		3.92
Forfeited		(97	)		3.84
Unvested balance at March 31, 2019		1,256		$	3.31