Don't get me wrong, I use social networks and media heavily. It's painfully obvious that all internet fads die eventually. Staying relevant is hard, because the internet has a serious attention disorder. It's a matter of time until the current concept of a social network will be replaced by the next myspace or facebook. Check out the article linked below for a well argued opinion.
So it's not that MySpace lost and Facebook won. It's that MySpace won first, and Facebook won next. They'll go down in the same order.
Douglas Rushkoff via CNN