I think that the essence of Groupon's value comes down to whether you believe that the $100 cost to acquire can in fact be consistently converted into the $300 at a rate that beats customer attrition, inflation, and interest rates.
Many of the stories in the media seem to indicate that businesses are getting burned by Groupon and that indicates that attrition is going to be a problem. Only Groupon themselves have a real idea of how large an impact this is having and how it will affect their business.
Early investors are cashing out pre-IPO which seems to indicate that insiders might have information that outsiders do not.
If I were a betting man I would bet that the insiders know that customer retention is an issue and the uncertainty is causing them to lock in more modest profits now instead of rolling the dice with everything post-IPO.