OSWeekly.com has posted an article on Social Networking Services

A social networking service is an online application specifically focused on building and verifying social networks for a specific purpose (whatever it may be). Many social networking services are also weblog hosting services. With the currently growing interest in online social networking services, social relationships in the real world are emerging in the Web and a user itself is gradually coming to play a central role in Web content. The emergence of user information from the real world into the Web has a great impact on information system.

Definition: Social Networking Services