Marc Andreessen's synopsis of the facebook platform/API is a good read for would-be social developers. Not that developers are social, er, I mean developers for social networks.

Favorite quote from Marc's article (emphasis mine):

"[W]hen your application takes off on Facebook, you are very happy because you have lots of users, and you are very sad because your servers blow up."

He's referring to iLike's explosion in particular, but it's good to remember that if you introduce something compelling in a huge, extremely viral environment, you can get a lot of traffic very quickly. Probably a good idea to have the phone numbers of a few content delivery networks ready.