Direct from Wikipedia: "A social network service focuses on building online communities of people who share interests and/or activities, or who are interested in exploring the interests and activities of others. Most social network services are web based and provide a variety of ways for users to interact, such as e-mail and instant messaging services...

MySpace and Facebook being the most widely used in North America;[1] Nexopia (mostly in Canada);[2] Bebo,[3] Facebook, Hi5, MySpace, Tagged, Xing;[4] and Skyrock in parts of Europe;[5] Orkut and Hi5 in South America and Central America;[6] and Friendster, Orkut, Xiaonei and Cyworld in Asia and the Pacific Islands."

http://en.wikipedia.org/wiki/Social_network_service