because it is a good way to keep a thin attachment to people who are just contacts, but people I don't want to loose touch with entirely.
That's one of the best reasons to be on it. I started using LinkedIn (free, not paying!) to get in touch with old colleagues; that's it, nothing more. Recommendations and invitations are for only people I absolutely know (I reject any others out of hand.)
For any social networking sites, it's the Thermodynamics of Humanity--crap and chaos will increase. AOL, Yahoo Message boards (social, financial, etc.), the garbage always builds up.
On that note, are only a few places I still follow that have stayed "fairly" clean. Joke as much as you want, but Slashdot has stayed pretty close to mission over the years. Groklaw is still pretty good. Motley Fool is still fairly new, but has hung on to it's central theme for a couple of years now.
Think of social networking sites like sex, or dating: Before you sign up, imagine that some Glenn Close nutjob is going to hunt you down and kill your pets, or some pimply teenager is going to show up on your door step 16 years from now at the family reunion shouting, "Dad! Mama tol' me you owed us for that fling all those years ago!"
If those kinds of problems are foreseeable, don't use the sites.