They're both good in their own way. The Facebook site itself is a lot better, and their developer API is awesome (it allows programmers like myself to write custom applications for it, and then users can add these applications to their profile). Their architecture was planned out much better (rather than the mess that MySpace maintains at the moment - Their new C#/ASP.NET code mashed into old, rotting ColdFusion code)
However, I use MySpace more, mainly because "everyone" uses it. I've got about 50 friends on Facebook, compared to 230 on MySpace.

And one thing about MySpace is they give you more freedom (you may insert your own HTML). Sure, a lot of people use this for "evil" (ugly, ugly MySpace pages), but it can be used for good purposes. As an example, I'm working on a layout that hides everything on the page and replaces it with totally different content (via <div>s).
Add me if you like?
MySpace:
http://myspace.com/daniel_1515Facebook:
http://c.zurl.ws/daniel15_facebook [redirect to my profile]