I've read through this thread and some of you have commented on the poor or unattractive design of the MySpace community.
I think you might miss the point here... the users don't CARE what it looks like. They want what it can do. And it is so much more than a forum. There are true "communities" set up that give them everything from brand new music (the music listings are truly huge) to blogs to event invitations and announcements to...
And, as far as the site design goes, the user can customize their account however they want. The pages they design may look awful, but they're having a blast designing them. Some of these people have never had any training or exposure to web design (and it shows), but this is their creative outlet.
I think that the fact that there are few limits is what is making this site so attractive. Worth $580 million? I don't know... possibly potentially more, since it appears that the user base grows exponentially every day.
As a musician that networks extensively, I can tell you that it is one of the hottest things ever for these bands to market their product. The band listings - with streaming audio (free to them) - have become HUGE!
Just my ten cents worth.
p.s. I don't think the design of this site is all that ground-breaking, either. But, I don't care what it looks like, I just want the communication tools available.