Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenue	$ 366,620	$ 291,609	$ 249,838
Costs and expenses:
Cost of revenue	106,268	78,878	62,479
Sales and marketing	149,773	156,814	103,344
General and administrative	60,557	56,967	45,873
Product development	108,414	82,145	55,707
Restructuring costs	29,420	2,043	4,468
Amortization of purchased intangibles	1,639	1,794	1,670
Total costs and expenses	456,071	378,641	273,541
Loss from operations	(89,451)	(87,032)	(23,703)
Other (expense) income, net
Interest (expense) income	(14,334)	(7,407)	22
Other (expense) income, net	(1,343)	1,213	(493)
Total Other (expense) income, net	(15,677)	(6,194)	(471)
Loss before provision for income taxes	(105,128)	(93,226)	(24,174)
Provision for income taxes	2,466	2,845	858
Net loss	$ (107,594)	$ (96,071)	$ (25,032)
Net loss per share of common stock:
Basic (in dollars per share)	$ (1.63)	$ (1.53)	$ (0.42)
Diluted (in dollars per share)	$ (1.63)	$ (1.53)	$ (0.42)
Weighted-average shares used to compute net loss income per share:
Basic (in shares)	65,888,450	62,593,026	59,203,400
Diluted (in shares)	65,888,450	62,593,026	59,203,400