As others have noted, it's become much more common. Partly because IPOs have become so uncommon, and the majority of large investments are private equity rather than public offerings. If they have no hope of a near-term IPO (because there was no IPO market for a while), founders can get frustrated locking up their time, effort, money, etc. into a company and start looking for acquisitions. If the investors don't want to see an early acquisition, and want the company to keep growing, the founders and investors may agree on an exit, or partial cash-out, for the founders that allows the company to keep growing.

It's not much different from the IPOs during the dotcom bubble: It makes the founders rich, and possibly pays off early investors, and transfers more ownership to the buyer (in the IPO case, the public is the buyer, in the private equity case, it is a VC or investment bank or conglomerate of the former).

In this case, it confirms for me that Groupon was built to flip. If it accidentally becomes a profitable company that lasts, I'll probably be as surprised as the founders (who, as you note, have already mostly cashed out; sure, they'll make more from the IPO, because I'm sure they still have some stock, but they locked in winnings already, and are mostly gambling with other peoples money from now on).