According to Wikipedia it is "a network service focusing 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." The most popular examples of these services are MySpace and Facebook.
These sites are integral part of our student's lives and could make a homework or classroom assignment more authentic or engaging. Twenty-first Century Educators need to be able to assimilate social networking into their classrooms so learning can occur inside and outside of the classroom.
Micro Social Network:

There is also a Ning community for teachers to share ideas with other educators (http://www.classroom20.com/ )
Excellent article on Ning from CNN