This is on the same line as knowing what the internet knows about you. At my college we found a website that seems to magically collect information from all sorts of sources. It doesn't get everyone, but a lot of people have found profiles about themselves that was compiled from a variety of websites which included pictures, addresses, phone numbers, and other unpleasant information that they didn't know was online. It is really creepy actually.
www.peekyou.comThus far I've been lucky. None of my info has found its way to this site and I credit that to my need for as much privacy as can be found online. A lot of my friends haven't been so lucky.
As for social networks as a method of choosing employees I have mixed feelings. I believe that what you do anytime represents who you are and you have to accept that, but I also believe that pictures and select words cannot truly give an accurate picture of a complete personality. I know many of my profile pictures, although not inappropriate in any way, would not make sense to anyone but my closest friends. I would have for an employer to not hire me because they didn't understand an inside joke.