Facebook

Facebook is principally a social network, but its the first stop for many people searchers due to its widespread popularity. By Facebooks count, 150 million active users frequent the site, about a third of which are in the United States. Even if you take those numbers with a grain of salt, thats still an enormous number of people who have put themselves out there to be found. Therein lies the strength of looking for someone on Facebook: By joining the service, Facebook users have essentially put up a big sign that says, Find me