This HBR article points out that rare is the company like Facebook or Amazon that achieves huge growth BEFORE demonstrating a profitable business model.

In both of those cases -- as well as companies that WERE profitable before scaling like eBay and Google -- the main asset demonstrated significant NETWORK EFFECTS.

Groupon has neither found a profitable business model, nor does it have network effects.

This could be a cautionary tale in the making.

Read more: http://www.businessweek.com/management/why-groupon-lacks-a-viable-business-model-08162011.html

The Bubble IPOs Convo also discusses Groupon and Zynga and questions whether these companies have demonstrated sustainable value: http://pandawhale.com/convo/99/bubble-ipos

Also, don't mean to be alarmist but Groupon might be running out of cash: http://www.businessinsider.com/groupon-low-on-cash-2011-8

Groupon's SEC filing disagrees.