I’ve worked with a lot of Social Networking sites directly, and indirectly reviewed a few others here and I see some common mistakes.
First, know your audience, and make sure they know who you think they are. Don’t build an adult site that appeals to children unless that is your goal (I hope it isn’t). If you are building a social site that has adult content make it VERY CLEAR who the site is NOT intended for.
Second – don’t ask me for too much information up front. If you have a “free” site that requires me to give anything more than a name and email address then I probably will pass.
Third, make it VERY clear what you offer – don’t bury things in marketing-speak. Plain English, tell me what you offer.
Fourth, let me SEE what you offer. Give me a demo. Let me taste your offering before I have to tell you anything about me.
Fifth – screenshots are important – it let’s us get a feel for the application, and it tells us a lot about how comfortable we will be in your environment.