BuddyPress.org

Also, consider that because BuddyPress uses WordPressMU which is compatible with most of the normal WordPress plugins out there, you have an existing free library of pretailored website features and upgrades that jump-start your development time.

On a large scale, using plugins like HyperDB and donncha’s cache plugin can help manage server load… Consider that wordpress.com uses a codebase very similar to WPMU to manage millions of users and blogs and activity, and it’s easy to see that WordPress is capable of handling a ton of pages and data… BuddyPress also now powers the http://profiles.wordpress.org pages too, so you can see how it handles tens of thousands of users there as well.