I have written many articles and ebooks on this subject. Here is an excerpt from a recent SEO article I wrote about social networks, social networking);
"There are many different social networks available including Twitter, Facebook, MySpace, countless forums, blogs, chat rooms, and a whole host of other social networking that hasn't even been invented yet. However, the concept of making sales from social networking is basically the same no matter what network it is.
Many people make the mistake of getting on too many social networks, trying to make sales on every one. They spread themselves too thin and wind up not making an impression on anyone. Other people make a presence on social networks by being a salesperson - going up to complete strangers and making sales pitches to them. This is an immediate turnoff and doesn't usually result in sales. So forget about trying to reach the masses through social networking and concentrate on one or two networks.
Start with some research. Start searching for your keywords and products and find out where people are talking about them. Then join in the conversation. Naturally you’ll have an opinion and naturally people will find out what your interest is. This is when sharing links - and talking about what you sell - turns into sales."