I think it probably translates to "Groupon is sustainable relative to a Ponzi scheme", which, even as a big Groupon skeptic for a long time now, I wouldn't challenge. A Ponzi scheme has no business other than selling ahead; Groupon does have a model for revenue. It may not work, but it really isn't fair to call it a Ponzi scheme. Bad business maybe, but not a Ponzi scheme. Ponzi schemes really ought to be reserved for actual Ponzi schemes and not be reduced to a mere financial insult.
A Ponzi scheme has no business other than selling ahead
Charles Ponzi arbitraged postage coupons (and resorted to selling ahead when that failed to turn profit). Strict application of your definition makes Ponzi's scheme not a Ponzi scheme.
All the sources I found online suggested that he started with the stamp selling, possibly legitimately, then transitioned into what we now call a Ponzi scheme, on a scale that could not possibly have been a result of the putative business. "It was uncovered that, in order to reach the level of investments that were made, 160 million coupons would have needed to be issued. In reality, only 27,000 had been. Soon afterward, Ponzi was arrested and indicted." [1] It wasn't a Ponzi scheme until he took the step into fradulent payouts.
As skeptical as I am about Groupon, it may work, in that it is not impossible by four orders of magnitude. Maybe they've got some brilliant pivot planned. Ponzi could never have made good using his putative business plan without fundamentally rewriting mathematics. But the real point isn't even that his plan was impossible, but that he wasn't executing the putative plan. Groupon is definitely soliciting businesses and issuing coupons.
