I'm not a fan of the big two, and I never heard of Bebo before. I heard of another network site Xanga or something where it's like anime based? I don't know, don't hold me to that 'cuz I didn't look too much into it.
Right now, and in the past I went to none of them. Not really an interest of mine.
In general, I prefer Facebook for one reason: Almost everybody I know has one. It allows me to keep up with the people I knew in high school and marching band from a few years ago.
I've never hated My Space. However, I do have one big complaint, which I fortunately haven't seen as much lately: Attention seeking. Primarily with pictures. I've seen people make a few bulletins a day asking for picture comments.
Constantine ripped the words out of my mouth and made a beautiful quote cake with them.
I don't use any of the sites now, though I have a profile on each. They all head the same way in the end but imo the force is strongest in Facebook. It's bombarded with image spam, useless apps and friday night vomit pictures but it's still the most in touch with the concept of social networking.