A friend of mine has run a website for several years called www.UserSearch.org and it is certainly worthy of a mention.

Designed as a tool to help look for various online profiles of any given individual, it can help assist in all manner of matters. It is aimed at the social networks of the modern age, including obvious sites like Facebook and twitter but also dating sites and the like. It used by investigators from many backgrounds, as well as by individuals who may just be trying to track down a long lost friend.

UserSearch.org homepage