Your analysis is wrong; here's why. The founders, employees, and other insiders of Groupon have already been handsomely paid off. For them, Groupon represents a free option. If it sells for $6 billion, great! But what if in 12 months it could be worth $10 billion? That's $4 billion more! And, compared to a site like YouTube, Groupon looks like a much more stable deal. They're already immensely profitable (something YouTube never was, and still isn't). The $5.3 billion offer, as a multiple of revenue and profit, was a surprisingly cheap offer for Groupon!

Maybe Groupon will look back in 2 years with regret. Or maybe they'll have an offer of several billion dollars more. For the people in the position to make the decision, the risk is worth it.

Do you know somewhere else with as good a chance as Groupon of making several billion more dollars in a year?