I have pretty much everything, although I use Bebo the most. In order of useage:
1. Bebo
2. MySpace
3. Facebook
4. This site
5. Live Journa (Although primarily it isn't a social networking site, you can still add friends and see what they're up to)
I use these primarily to keep in touch with friends and to look for new friends with simular interests etc. Although some like Bebo and Facebook have Applications which are fun, some are stupid :P
MySpace I use to check up on my favorite bands rather than look for new friends but either MySpace or Bebo are good for that. Neither one outways the other, I just prefer to use Bebo because in my opinion tiz just easier :P
Hope I was helpful :P