It's really just semantics at this point. There is a lot of evidence here that suggests systematic misleading (if not out-right defrauding) of investors. So you while you are technically correct, I think what Groupon has done is in the spirit of Ponzi even if it is executed differently. What's happened here is more than just a bad business plan executed honestly producing poor results. Just because we don't have the exact word for it doesn't make it any more ethical.

It is that money sitting in the account that gives Groupon the cash flow it needs to hire more sales people to grow the revenues further, but remember always that a % of that must be returned to the customer at a future date. Where it falls apart is when your new sales can no longer replenish the client account (no it does not sit in a trust or escrow, but in company's general operating account).

So what happens when Groupon declares Bankruptcy? All that money sitting in the account is gone, customers get NOTHING back. Yes, this is indeed a Ponzi scheme because the last customers in get nothing, even though they are owed the %.

But no, not a ponzi scheme at all.