Revenues are nothing without earnings though. Facebook spends more than it takes in, which can easily be accomplished by anyone (especially as of late!). The valuation is purely on a bet that in the future Facebook will be have more revenue generating capacity. It's a bet, albeit a big one. With so many users it doesn't take a whole lot of profit per user to get to big revenues, but it's far from a sure thing that Facebook will end up worth anything substantial.