a) I thought the whole AAA thing had been found to be bankrupt? (Honest confusion.)

b) How many companies who turn down seemingly-huge acquisition offers turn out later to be 'just' whatever they were doing at the time of the offer? Most startups at any stage don't stay fixed at doing 'just' whatever they're doing at that stage.

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.

"A bird in the hand is worth two in the bush."

And said bush is surrounded by a ring of fire. And explosives. And zero humidity.

Regardless, we don't have all of the data - hard to tell from outside

True, we don't have all the data, but we do know that those two bush birds are already producing millions of dollars in profit a week.