50 million dollars for a web app? Is facebook even profitable yet? Call me a cynic but i don't think their "billion dollar company" will withstand the sands of time. Just wait for the next paradigm shift.
Last I heard, they were expected to post $550M of revenue in 2009. Any insider you talk to says it would be trivial for them to become profitable, if they wanted to. And, in fact, they may be quite close to doing so in their current plan.
But then again, I don't have any inside information either. One thing's for sure, though: Bringing out the tired "Facebook is overvalued" and "Facebook will never turn a profit" lines is seriously annoying.