That $15 billion number was inflated. It was valued at $10 billion in January 2009, at which point it had "made a penny directly in advertising". It also had 175 million users the following month, or 17.5x the number of users of Pinterest.

http://online.wsj.com/article/SB124335674958054943.html

http://www.insidefacebook.com/2009/02/14/facebook-surpasses-...