My laptop hates MySpace, so I rarely log in to it, usaully only at work.
Facebook is to keep in touch with people I talk to online, and a few friends.
Twitter is for fun, to keep people updated, to follow people I find interesting, and to share cool pictures. With the 140 character limit, I don't have to worry about long rants about stupid stuff. Only short rants about stupid stuff.

Facebook I also do FarmTown, but I turn down or block most other applications.