How are Social Networking Websites Able to attract so Much Traffic?

Social Media
For some people it is very surprising that how are social networking websites such as Facebook, Twitter, Myspace, and LinkedIn able to attract so many people? For people who do not know this fact; yes, social networking websites are the websites which get the most hits on the world of internet and attract the most traffic. These social networking websites receive hundreds and thousands of hits daily and are attracting all the traffic from internet around the world. There are billions of users who use Facebook, Twitter, LinkedIn and other social networking websites daily.