Groupon changed from "gross revenue" (which did not deduct money paid to merchants) to "net revenue" (which did). I believe they did this willingly because they were going to IPO, not because they were "slapped" or due to GAAP.

Living Social is private and can do whatever they want, but they probably are using net if they're comparing themselves to Groupon. Otherwise they're making themselves look half the size when they are really a quarter, which I suppose could be purposeful.