Before you ask "How" you need to successfully answer "Why": Why do you want to build one? Who's going to use it? What makes it different from the others? Will you get enough people to use it to make it different than the others? Does it fill a niche that others are missing? What kind of people do you expect to use it?
Google developed a very competent social network but with competition from Facebook very few people use it. And that's the problem with social networks: All the content is user generated; the only reason people go on there is for other users. If you don't have users you won't gain them.