The Deal Professor's take on how bankers conspired with Groupon and Zynga management teams to play with financial numbers is a must-read.
"Grouponomics and Zyngametrics, but Few Sound Numbers" NYT
Quite apart from the dubious business models of these companies, people who might consider buying into the IPOs should consider the signals from management: at Groupon, the investors who provided funds in 2010 have already cashed out previous investors (what would the motivation be for new investors to fund the exit of previous owners?); and at Zynga, despite wanting the public to buy ownership of the company, these new owners would have no effective voting rights since insiders will retain almost all of them (investment without representation!).