That's a little weasely, because the people calling Groupon a Ponzi scheme tend to talk about how minimal their competitive advantage and technological "moat" is. Google can build stuff. Why'd they try to buy this?
It is usually easier to modify working code than to write something from scratch, if my suspicion is right, I expect Google will launch its own Groupon-like service integrated into its other offerings eventually.
Google isn't infallible, they are mortal. I think most likely they were willing to overpay because they could afford it and are desperate to get into "social".
Also, Groupon is a ponzi scheme because they use new investor money to pay old investors--not because they don't have a moat, which they dont-- no patents, 8000 employees and how do you scale with that many employees, a list of merchants and user emails.
At best they end up looking more like Avon.