I'm a facebooker personally. The layout is clean, simple and with the new look it's impossible for someones profile to bring your browser to a halt with tons of videos and gifs etc.
I used to use MySpace. It's strongest asset is also it's biggest liability and that's the sheer amount you can customise the profile. Some people use it to make incredibly personalised, unique and interesting profiles. Others make you want to rip your eyes out with a rusty spoon.
Something I've also found is that MySpace is *very* spam happy. Thousands up thousands of chain mails are everywhere. Facebook had an app-spam problem when they first came out but these days I barely ever get an unwanted app-invite.
As for Bebo, it's strange. In England at least, it's the Chavs (I think you americans would call them "wiggers") who use Bebo, everyone else is on Facebook. So Bebo is a no-go zone simply because everyone on there is a world class... annoyance