At least some of those companies have actually turned a profit and stand a chance of maintaining one. I have my doubts about Groupon's and Zynga's medium-to-long-term chances, but I've been wrong before.
there's going to be a lot of young, early employees that are able to cash out
Early employees will probably not be able to cash out for at least six months; that's the standard waiting period after an IPO. Ironically, people who get friends-and-family deals pre-IPO are the most likely to get rich quickly (if they sell on an early uptick).