Ten-year Treasury yield is 3%. $6 billion invested today for ten years would return $8 billion. Their valuation could be several billion more than $8 billion in less than 2 years. You might be right about the coupon sites being a novelty, or maybe Facebook, Twitter, or Google (or, some as yet unknown competitor) will "monetize the long tail"...

But... the founders, employees, and other insiders of Groupon have already been handsomely paid off. For them, the chance at several billion dollars more in a few years time is worth the risk of rejecting the Google deal.