Redjake, yep, I admitted my bias and I stated why I wouldn't want to use these sites.
But that doesn't mean I didn't want to know why other people used them! (I'm an insatiably curious social scientist, after all.) Hence the OP.
We'll see if I ever get around to posting on one. TFP is enough for me, and I like being anonymous for now. As I see it, Facebook, etc. is like creating a resume for social participation... and I'm not interested in broadcasting myself that way. If I want to get in touch with old friends, that might be one motive for joining... but then again, I am already in touch with all the old friends that I *want* to be in touch with!
I do understand more now why people use Facebook, etc... so I thank all the respondents to this thread.