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.
You know what screams "bubble" to me? Everyone defending valuations by comparing the company du jour to Facebook.
It's like arguing that Webvan wasn't overvalued because Amazon.com has been successful. Unless a seriously meaningful connection between Facebook and Pinterest is drawn, the comparison is meaningless.
http://online.wsj.com/article/SB124335674958054943.html
http://www.insidefacebook.com/2009/02/14/facebook-surpasses-...