The Google crawler counts the link of the social networking sites or blogs to get the internet searches. The social networking sites or blogs can divert visitors to main sites. Any types of content or entry over the networking entry count on ranking to site.
Social Networking sites owner submit the site to various directories like reddit and dig. Many sites provide review of social networking sites and blogs so that they can get maximum traffic to their parent site. You need to look at the specific target audience to attract the traffic for the community.