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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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