We've been discussing Facebook, Myspace, and Twitter in my CIS 101 classes recently, under the guise of social networking. Business folks refer to the collection of technologies like social networking, wikis, etc. as "Web 2.0" content - more interactive Web pages.
Aside from the obvious uses, there are other things that these sites are used for. One thing that you have seen in certain areas is police departments asking for help. For example, in the link below, a New Zealand police department caught a thief by posting a security image on Facebook. This is an article from last year, but it's still an interesting application of social networking.
Link to Story