I ran across a website that attempts to do a mash-up of the various social networking sites out there to provide one place to search for people. www.spock.com From what I can tell they heavily use both LinkedIn (a professional network) and MySpace (a social network) to gather their data. There is obviously a lot of repetition in their results, but if you are looking for someone and have some clue as to where they might be or have been, this search could be useful in tracking them down. You can search on just about anything that might be in their profiles on the various sites, so that opens it up to various search angles.

From a technical viewpoint, this probably was not very difficult to pull off. A little screen-scraping and your done.