I'm going to disagree with a lot of these other comments. What does your simple social networking site do? Is it a close clone of another site? For instance, Digg for cat lovers? Twitter for Amiga holdouts? Groupon for Waco, Texas?

There are commercial and open source clones of most of these sites out there of all kinds of quality. That doesn't actually matter so much - what matters is that you can quickly get a prototype of what you want to build in front of your potential users/customers.

Swap out some colors and images and make your clone script look close to what you want to build.

If your idea takes off, then you can start looking at building from scratch.

Hope this helps!