Groupon obviously has an early-mover advantage, although it doesn't seem insurmountable, but their technical advantage doesn't seem enormous. I don't mean to undersell what they've done and fall into the "I could build X in a weekend" trap, but what they've accomplished technically seems easily repeatable in a year by a company of Google's talent, if not less.
Their early-mover advantage has established them as the definite leader in the area, but again, it does not seems insurmountable. There isn't significant vendor lockin (the vendors are constantly changing), nor customer lockin, (other than having to sign up for a new site). There is a clear avenue for competitors to break in, by offering better terms.
Basically, they are printing money by being a middleman. Funny, i thought the internet was supposed to eliminate that kind of business. They are certainly worth billions, as they are enormously profitable, but 6 billion seems excessive. What this says to me is that Google sees locally-based commerce that occurs online (bad phrasing by me) as a critical space in the coming years, and they would rather significantly overpay and jump ahead rather than risk being lapped.