Facebook got a $15B valuation from investors including Microsoft way back in 2007 [1] when they had only 50 million users (MySpace had 100 million users) and absolutely no advertising infrastructure in place whatsoever; they got this valuation and made $240 million in investor funding before they'd made a penny directly in advertising.

Meanwhile, Pinterest already has amassed well over 10 million users in a very short time, sees enough hits daily to be a top-ten social networking site, and already has a number of advertising arrangements, particularly with retailers like the Gap and Nordstrom.

If this is a bubble, it's been around for a while; and this doesn't seem like the most sterling example. If anything, this is less stunning than valuations of tech companies we've been seeing for many, many years.

[1] http://www.wired.com/techbiz/startups/news/2007/10/facebook_...