Facebook has always been more popular than MySpace. FB is a more personal, friendly and less overwhelming than MS. The majority of MS accounts nowadays are bands, clubs, pubs and other companies and organisations trying to promote there services and products.
Bebo is to be sold or closed, Myspace is making huge financial losses, but then, so is facebook, face it, when you get that big, it's hard to not be successful.
MySpace is full of kids posing in ridiculous pictures, with nearly no clothes on trying to get as many picture comments as possible to make them feel popular as they have the same level of human social ability as a fish. It is also a crap website, when they finally managed to come to Web 2.0, it was only half the website and still is really. Apart from that, it is barely usable if you don't continue to get 'Unexpected Error! Has been reported to Technical Team!'... yeah right.
Facebook on the other hand is brilliant, it's like an online communities for families, friends and colleagues to network and keep in touch. Fantastic stuff.
Myspace is better if you want to promote your band, music, film, but so is Youtube.
Facebook sort of reminds me of LinkedIn, but with more apps, and users. It is unfortunate that Facebook does not allow users to install custom template, and use HTML and Javascript codes in user template.
I don't really use either. I mostly use them as a social bookmarking site or to login to other sites via Facebook Connect.