Facebook was virtually unknown when it was limited to a small number of US colleges. When it opened its doors, it grew, because people who didn't go to college had friends who used facebook, so they made a facebook too. That's how these sites grow, but they need a "critical mass" before they grow at the rate that Facebook is growing.
Myspace has stopped growing for two reasons:
(a) It's got 180 million users, so it's likely that most people who have friends on myspace are already on myspace. I.e. there's no-one left to sign up!
(b) It's becoming increasingly unpopular, as people who used it two years ago start growing up and going to university, where Facebook is king, and as more and more 14 year olds use it and turn it into something rather more childish than facebook.
The same thing happened with ebay -- it's been around for over a decade, but it's grown faster in the past 2 years than in the previous 8 combined. That's because it reached "critical mass", especially outside of the USA.
