Social networking sites grow and succeed because they create or help to create relationships.
Like minded people flock around groups and clubs because they find there people who think like them or have the same or similar interests.
Have you heard about the Angry birds game? In fact it is a great success. However, the people who play and like this game, they like to know other gamers experiences and join groups to share those experiences.
Fan clubs and groups grow in direct proportion to other peoples interest around the same movie star, or singer, or soccer player orwhatever. Same likes and interests create relationships and that is reason social networking sites are so successful.
Marketing is based on relationships. All the following you build, each of these is designed to create and foster more intimate relationships with people, in some cases, people you might not have met any other way.