They are popping up at an ever-increasing rate. By socializing with others on the net through these sites, everything you join or participate in gives you more people to see your site. And, if you're selling stuff from your site, you'll want to go to the places (interest groups) on the social site where people are interested in what you're selling.

Web traffic is the amount of data sent and received by visitors to a website. This necessarily does not include the traffic generated by bots. Since the mid-1990s, web traffic has been the largest portion of Internet traffic.[1] This is determined by the number of visitors and the number of pages they visit. Sites monitor the incoming and outgoing traffic to see which parts or pages of their site are popular and if there are any apparent trends, such as one specific page being viewed mostly by people in a particular country. There are many ways to monitor this traffic and the gathered data is used to help structure sites, highlight security problems or indicate a potential lack of bandwidth.