Scott Rafer hits it on the head:
If Jonathan had instead of found a way to accommodate hundreds of thousands passionate users who were doing a ton of free work on his behalf, I don’t think we would have ever heard of MySpace. The pets example is the clearest. In the Fakester purge, Friendster is reputed to have deleted 300,000 pet accounts. As a spoof, one of the funniest guys I know set up Dogster and Catster. It’s still funny but no longer just a joke. The two sites now have 150,000 active pet owners, a bunch of unregistered traffic, and a great little business that does whatever the heck pet owners want and gets paid nicely for it.
Jonathan [or whoever was making the decisions] wanted the site to be a dating site. However, the users wanted it to be a place where they could express their creativity and humor by creating fake accounts and other TOM foolery.
I was actually thinking about this yesterday, but from a feature standpoint. Friendster keeps adding stupid features like horoscopes and sending out lame emails that says, "We miss you. We have new features."
But, what myspace has that Friendster doesn't have (the last time I looked) is the ability to post html all over the place. People can customize their profile pages like it's geocities. They can leave comments and send bulletins in html. They can set up event invites and groups, etc, etc and do the same thing. It enables people to express their creativity or lack of it. (And to pimp their stuff.)
The lesson to be learned from all of this:
Your users might find other uses for your creation. Let them run with it.