I get a lot of questions about social networks, so I spend a lot of time explaining them. I was recently pointed to this video on YouTube which, even though from 2007, still offers a pretty good overview of social networking.
The video is by a small company called CommonCraft. Their main business is in the creation of video content for the Internet. With this posting, I think they have done a very nice job in explaining what social networking is for the lay-person and I intend to point people to their website to view it when I give talks or get questions on the topic.