"Social Networking" - hooking up with friends and acquaintances
I find MySpace to be too "spammy" and you get "friend" requests from people you've never heard of. Facebook has a neater, cleaner look, and I think it's more user friendly while having more that you can actually do there.
MySpace is overrun by teens, while Facebook was originally made for college students, if that helps to understand the difference a bit.
YouTube as far as I am concerned is mainly about posting videos, its primary purpose isn't social networking like the others.