Originally Posted by WiredGuy

This isn't really a job for a designer, more of a programmer. We're talking about a backend database, interactivity between members and most importantly, scalability. These are things most designers don't have a clue on how to do. You need a programmer / project manager for something like this.

I'd also suggest looking for an existing product than start from scratch, do a few Google searches for myspace / facebook clones, they do exist.
WG