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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 10, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Concentration Risk [Line Items]
Deferred offering costs		$ 2,627		$ 2,627		$ 0
Deferred offering costs incurred but not yet paid				1,076	$ 0
Advertising expense		7,685	$ 6,460	19,751	$ 10,495
Volunteer program, one-time payment to contributors	$ 5,098	4,220		4,220
Volunteer program, transaction fees	$ 526
Volunteer program award reserve		(878)
Contributor awards		$ 577		$ 577		$ 0
Accounts Receivable | Customer Concentration Risk | Google
Concentration Risk [Line Items]
Concentration risk, percentage				66.40%		28.90%
Accounts Receivable | Customer Concentration Risk | Apple
Concentration Risk [Line Items]
Concentration risk, percentage				18.30%		47.80%
Accounts Receivable | Customer Concentration Risk | Stripe
Concentration Risk [Line Items]
Concentration risk, percentage						13.80%
Revenue Benchmark | Customer Concentration Risk | Google
Concentration Risk [Line Items]
Concentration risk, percentage		29.30%	26.50%	28.80%	27.60%
Revenue Benchmark | Customer Concentration Risk | Apple
Concentration Risk [Line Items]
Concentration risk, percentage		51.60%	50.00%	50.90%	52.10%
Revenue Benchmark | Customer Concentration Risk | Stripe
Concentration Risk [Line Items]
Concentration risk, percentage		10.30%	10.10%	10.30%

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