If you plan to code a cms from scratch, you're completely nuts or a fucking genius, or both ;-) I would never reinvent the wheel, unless I have a completely new idea of wheel.
Drupal is good and has a great supporting community, but If I were to build a "social network type site", i would look for something more specific. there's stuff around that can get you started with almost no customization.. take a look:
http://www.google.de/search?q=social+network+scripts