Total estimated stock-based compensation expense for employees and non-employees, related to all of the Company’s stock-based awards, was as follows:

	Three Months Ended
	March 31,
	2022		2021
	(in thousands)
Cost of revenue	$	26	$	397
Selling and marketing		398		331
Research and development		283		249
General and administrative		830		809
Total stock-based compensation	$	1,537	$	1,786

The following table presents the stock activity and the total number of shares available for grant as of March 31, 2022:

	Options Outstanding
	Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding at December 31, 2021		1,739,240		$	7.72	$	7.02	$	25,542,823
Options Granted		-			-
Options Exercised		(47,078	)		7.67
Options Forfeited		(5,198	)		16.27
Outstanding at March 31, 2022		1,686,964		$	7.70		6.64	$	23,231,263
Vested and expected to vest at March 31, 2022		1,666,854		$	7.75		6.62	$	22,967,200
Exercisable at March 31, 2022		1,075,419		$	7.08		6.15	$	15,595,467

Stock options are time-based and the majority are exercisable within 10 years of the date of grant, but only to the extent they have vested. The options generally vest as specified in the option agreements subject to acceleration in certain circumstances. In the event participants in the plan cease to be employed or engaged by the Company, all vested options would be forfeited if they are not exercised within 90 days. Forfeitures on option grants are estimated at 10% for non-executives and 0% for executives based on evaluation of historical and expected future turnover. Stock-based compensation expense was recorded net of estimated forfeitures, such that expense was recorded only for those stock-based awards expected to vest. The Company reviews this assumption periodically and will adjust it if it is not representative of future forfeiture data and trends within employee types (executive vs. non-executive).

Aggregate intrinsic value represents the difference between the estimated fair value of the underlying common stock and the exercise price of outstanding, in-the-money options. The aggregate intrinsic value of options exercised was $0.5 million for the three months ended March 31, 2022.

The Company uses the Black-Scholes option-pricing model to estimate the fair value of options granted as of the grant date. There were no new options granted during the three months ended March 31, 2022. The total estimated fair value of employee options vested during the three months ended March 31, 2022 was $3.1 million. As of March 31, 2022, total unrecognized compensation cost related to non-vested stock options granted to employees was $2.5 million, which is expected to be recognized over a remaining weight average vesting period of 1.8 years.

	Shares			Weighted Average Grant Date Fair Value Per Share
Nonvested restricted stock at December 31, 2021		788,454		$	16.81
Granted		42,900			23.21
Vested		(29,400	)		20.88
Shares forfeited		(4,587	)		18.75
Nonvested restricted stock at March 31, 2022		797,367		$	17.00

As of March 31, 2022, total unrecognized compensation costs related to the nonvested restricted stock awards was $10.2 million, which will be recognized over a remaining weighted average vesting period of 2.3 years.

As of March 31, 2022, the Company had 134,000 performance-based restricted share units outstanding. The vesting of performance-based restricted share units is determined over a three-year period based on (i) the amount by which revenue growth exceeds a defined baseline market growth each year and (ii) the achievement of specified tiers of adjusted EBITDA as a percentage of net revenue each year, with the ability to earn and vest into such units ranging from 0% to 200%.