A reasonable point, but Google has several years' worth of results under its belt. When the IPO was conducted I think the company was valued at about $25 billion; now it has net annual earnings about $6-7 billion - so while the P/E ratio is still high, at least it's based on actual numbers. I'm not saying FB couldn't be worth even more than Google - it's just that since they've never had to file earnings statements etc. yet, all such estimates are far more speculative.
http://www.google-ipo.com/
http://en.wikipedia.org/wiki/History_of_Google#Financing_and...
http://blogs.wsj.com/digits/2010/03/04/investors-bet-on-pric...