from: http://webtrends.about.com/od/socialnetworking/a/social_network.htm
MySpace: Started in 2003, MySpace was a driving force in popularizing social networking and still maintains the largest userbase.
Facebook: Founded by Mark Zuckerberg, Facebook was designed as a social networking site for Harvard students. After spreading from Harvard through the university ranks and down into high school, Facebook was opened to the public in 2006.
Hi5: A fast-growing social network with a strong base in Central America, Hi5 has over fifty million users worldwide.
Ning: A social network for creating social networks, Ning takes the idea of groups to a whole new level. Created by Marc Andreessen.
Flixster With a tagline of “stop watching bad movies,” Flixster combines social networking with movie reviews.
Last.fm Billing itself as a social music site, Last.fm allows members to create their own radio station that learns what the person likes and suggests new music based on those interests. In addition to this, you can listen to the radio stations of friends and other Last.fm members.
LinkedIn A business-oriented social network, members invite people to be “connections” instead of “friends.” Linkedin is a contact management system as well as a social network, and has a question-and-answer section similar to Yahoo! Answers.
Xanga A social blogging site that combines social networking elements with blogging. Members earn credits for participating in the site and can spend credits on various things such as buying mini-pictures to post in the comments of a friend’s blog.
Badoo: Based in London, Badoo is one of the top social networking sites in Europe.
Migente: A social networking site targeted at Latin America.
Orkut: Originally created by Google to compete with MySpace and Facebook, it has mainly caught hold in Brazil.
Studivz: A German version of Facebook with a strong audience in students.
Web 2.0: Who, What, Why, How 