If their revenue projections are accurate, then $10bb is not that crazy. They also have 200 million users and each user has some value - though I have no idea what it is.

And I know people will say "but they're not accurate with their projections." If you were projecting revenue or growth for your start up you'd be saying the same things Facebook says to their investors, showing the numbers that make your case. You cannot blame them for the valuation when investors seem to agree. No one is forcing them to invest.