It is ethical for colleges, parents, or employers to search social networking sites for information about people?

According to juggle.com 73% of users say YES.

Some tools to help get you started with nosing around social networking: Spy, SamePoint, SocialMention, WhosTalkin, yoName,  snitch.name,  Folowen,  Spokeo, Wink  and a ton of others…

And there are courses out there as well, for example -
Social Networking Sites: Investigative Tools and Techniques, …

There are probably more then a few good business opportunities to be had in providing social networking / media investigative tools (as opposed to social media monitoring).