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.
The web is built on links, and web users routinely use links to find the information they need. When a site links to your site, web users follow those links, resulting in web traffic, or referral traffic. When these backlinks, also called inbound links, come from high quality sites, they also tell Google your content is authoritative and trustworthy. This is important because inbound links from respected sites are an SEO ranking factor, so they can boost organic traffic, too.
Not all web traffic is welcomed. Some companies offer advertising schemes that, in return for increased web traffic (visitors), pay for screen space on the site. There is also "fake traffic", which is bot traffic generated by a third party. This type of traffic can damage a website's reputation, its visibility on Google, and overall domain authority.