If you plan on getting a lot of load on the site, then you’ll need to start off with a dedicated server, and be prepared to have a lot of servers vary quickly. You’ll need to build the system with the ability to scale in mind.
The largest Social Networking sites like MySpace and FaceBook use thousands of servers (if not tens of thousands of servers). These sites have their own data centers dedicated to running just their servers and networking equipment.
To start with you’ll need a web server and a database server. Any little problem that you find will be amplified once you start getting a few thousand people hitting the site every hour.