MySpace is more of a musicians/music fans sorta site. There's been a lot of hype from people using MySpace the last few years - Tila Tequila and My Chemical Romance come to mind, though there are many others. Essentually, MySpace is a social network of your "friends" both from RL and 'teh intarwebz', though it's focus seems to be moreso for people that are online and fans of music etc etc.
Facebook is more of a social networking site - you add people to your friends list and they can veiw your page, which lists most of the common information about you. It started on USA some college/university campus (forget which) and since open it's gates to any user of any age from any background and from almost any country, Facebook has become huge. It's useful for keeping track of old friends and relatives and such who you might not talk to often.
MySpace, due to the ability to edit pages via html and the like, can be very unstable, with page crashes, downtimes, accessing problems and the like (I've heard viruses as well). I've had various friends complain of that and more. Facebook, OTOH, is much more stable and
by far much more user friendly, if a little more "vanilla" than MySpace.
I guess it depends what you want; I never really bothered with a MySpace asides from a very basic page - and since I've been in a band, that page as well. Facebook, OTOH, is a better site and more suited to my needs - you can post messages and blog just and you might do on a MySpace, post albums of up to 60 or so pictures, connect with friends and chat back and forth between public "wall posts" and private messages, join interest groups and chat on the (fairly boring and frustrating) forums and keep tabs on all of your friends. It's a fun little site that I visit daily
cheers,
--N
