BuddyPress, the social networking software for Wordpress, was originally only compatible with WordPress Mu.
BuddyPress 1.2 just added compatibility with "normal" Wordpress, which creates opportunities for an enormous number of web sites.
Unfortunately, there are almost no existing Wordpress themes which are compatible with BuddyPress. So, the BuddyPress team just released the BuddyPress Template Pack, which helps you modify your existing themes to support BuddyPress.
If you have a WordPress site which receives more than a thousand visitors a day, you should really look into adding community features to it with the new BuddyPress.