All social networking sites need a social networking policy that you've created. Social networking policies outline the do's and don'ts for your site, ranging from what you consider to be harassment, to how your users are expected to behave.
Successful social networks revolve around a compelling object or activity that is the basis for a real connection, said Aaron Levie, chief executive of Box.net, a website that combines social networking, content management and online file storage.
Make sure there's a compelling reason for people to return to your site. Will they share music, photos, videos or travel tips? "Pick something that resonates with the group of people that you want to engage, and make sure your social network makes that activity very, very easy to do.
Social networking sites are always built with the same basic principle: the goal it to provide online sharing and organisation tools for the members of the community. If you are starting a website for an existing community of which you are a part of, you should have a pretty good idea of what the people need.
Ok, knowing that you have most likely typed something like "how to start a social networking site", I am going to assume that you are not a developer. This implies that you will not be able to shake a social network out of your sleeve in the next couple of days.
All is not lost however. First of all, get help. Find a group of people who like the idea, and put your minds together. If you can get financing from somewhere, go for it. That's what you'll need to start developing a site that does exactly what you want it to do, with security, powerful servers and everything!