BuddyPress.org

The one thing that is truly network-wide about BuddyPress is that posts from every blog on the network will get tracked in the activity stream by default. If you turn off blog tracking (Dashboard > BuddyPress > Component Setup) then it’ll be functionally equivalent to having BP installed on a single blog. If you want to exclude single blogs, you could write a little plugin that unhooks the bp_blogs activity actions, and activate it on the blogs that you don’t want tracked.