Just want to point out that in many industries a revenue multiple of less than 1.0 is expected [1], such that generating greater than $15 billion in revenue does not automatically result in a company worth $15 billion. That being said, FB's net assets are $50 billion (and it's market cap is much, much (holy cow it's alot) higher), so clearly those that got in at a $15 billion valuation are very happy right now.

[1] http://pages.stern.nyu.edu/~adamodar/New_Home_Page/datafile/...