Entrepreneur.com has a decent article on how to use social media to help market and build your business. The article is a little focussed on MySpace but the principles are still good. Social media creates great opportunities to network and meet new people who could bring a whole new group of customers and visitors to your site and business that may not have been possible before.
Although there is quite a big emphasis on the sites themselves and the tools they employ, it is a good idea to bear in mind that these are really just new ways to do what people have needed to do for ages to develop a following in the world – network. Sites like MySpace and LinkedIn just make it so much easier to reach out across your city or to the other side of the world and connect to new markets and, perhaps more importantly, other people who can connect you to even more people.