Social networking sites, a good idea?

For a while now employers have been using social networking sites to find out more about job candidates. Well now it looks like universities might be using them to see if their students are engaged in activities that the school finds unacceptable. I really do not have a problem with this, as my father-in-law would say "when your dumb, you suffer". Why do people not think first when posting stuff on these sites. Once there it becomes public knowledge for everyone to consume. If you don't want your parents, employer or school to know about it don't post it online. I remember when I was in high school MTV was in town for spring break every year. The school would tape the shows and then watch them to see if any of the students where there. Did I miss a day or two to be at the beach with MTV, of course I did (sorry mom) but I did not try and get in front of the camera. That would have been dumb.