Nancy's got some great coverage of artists who are discovering that they've got to migrate their audience away from MySpace onto their own websites so they can really start to benefit from their relationships with fans. Here's one funny story of an artist who cloned his whole MySpace page on his personal domain, and here's a followup about Tila Tequila -- who's discovering that it's one thing to have 1.5 million MySpace friends, and it's another thing entirely to get them to buy something.
At the same time, I'm hearing rumblings from the students I work with and speak with that MySpace is "so over" and Facebook is where it's at. (Just looking at my alma mater's student newspaper is proof of that. They're covering stories about how campus safety busted some underage drinking parties after seeing some Facebook photo galleries, and they're building Facebook links into all of their articles.)
MySpace is still a very powerful tool, but it's not the only tool that you have at your disposal. And there's a strong chance that users will become more annoyed with it and move on. Build your own site with an easy way to get your audience to migrate across.