This is a technicality, but there's a difference between a Ponzi scheme and a pyramid scheme. A Ponzi scheme needs an information asymmetry: investors don't know that their money is being used to pay off previous investors. A pyramid scheme is open about the way it works and the pyramid nature, but people invest anyway.
Since Groupon's investment and revenue strategies are known to all, this would be closer to a pyramid scheme than a Ponzi scheme.
I was wondering this too and spent the past 20 minutes reading the (very good) Wiki articles on both Ponzi and Pyramid schemes. I'd agree this is closer to Pyramid than Ponzi.
I thought a pyramid is multi-level marketing. I get n people to pay me something while they each get n people and so on until there aren't any other people entering the scheme.
It's easier to blame the participants in a pyramid scheme than in a Ponzi scheme?
Now that this stuff is becoming so common, I guess the approaches of "find the bigger fool" and "have your alibi in hand" are being honed to science...
Since Groupon's investment and revenue strategies are known to all, this would be closer to a pyramid scheme than a Ponzi scheme.