BuddyPress.org

Short answer, it was written to use anything other than the main WPMU install domain. Even though the MSM plugin allows you to almost run “additional” WPMU instances (virtual), the plugin suite doesn’t work that way. You would have to actually create another install for social.domain.com and run BP or hack the code up which I am sure would be a wreck. Don’t think there is an easy way around this one I am afraid.