BuddyPress.org

I took a hard look at Elgg for the site I’m setting up at the minute. I actually started the site with Elgg, but switched over to BP in the end. Elgg, as a standalone social network engine, might be the perfect choice for some people, but the learning curve is a lot steeper than with WP, especially if you already have some WP experience.

And the fact that WP and WPMU are gonna be merged means that WP as a whole is gonna be a lot stronger. And you just need to look at how many users/blogs WP.com can handle, so with the correct server setup your BP site has no limits scaling wise.