Groupon has had pretty huge revenue growth and is currently bringing in some hundreds of millions of dollars a year in revenue.

Skepticism about the ability to maintain that growth rate, and the suspicion of many (especially here) that they're burning bridges with the companies they work with put a high valuation in doubt, but that's a matter of opinion at this point. Evidence that Groupon is having any trouble finding partners to offer coupons is so far anecdotal.

I never thought there'd be this much money in coupons, but so far it looks like it's a great business model. It wouldn't shock me if those valuations were true.