Social networking focuses on building online communities of people who share interests and/or activities, or who are interested in exploring the interests and activities of others.
Depending on the website in question, many of these online community members share a common interest such as hobbies, religion, or politics. Social Networking are sites like MySpace, Facebook, LinkedIn, Twitter and others.
Why would anyone want to join a social network?
The big reason social networks are so popular is that they allow people to keep track of their friends easily. Most social network services provide a variety of ways for users to interact, such as e-mail and instant messaging services. The idea is that you can have one place to visit to keep up with everyone in your network.
In SEO terms these sites allow you to post your ads and promote your products (not spamming) and attract more traffic. You might want to find ways to communicate with people, build groups, joint different networks.