This is simple to explain, Facebook isn't anywhere close to its revenue potential, many people think it has the capacity to make Google type money as brand advertising moves online. If you look at their stock price in that light its not overvalued at all. Google's 2004 P/E was around 100, even in 2008 it was 46.

EDIT: oops, I pulled revenue not earnings. Its irrelevant in any case. By the time Facebook IPOs their earnings should be on par with Google's IPO earnings.