I am using Networks+ (same thing as WP Multi Network) as well as BP Multi Network mu-plugins to create separate segregated BuddyPress social networks for each sub-site. It has really worked out great. Basically, all the groups, group activity, group members, etc. are unique for each sub-site. The only thing it does not segregate it for is when you click the All Members tab or are viewing widgets like "Recently Active Members" or "Whose Online" since these types of things always pull from the root user list from the entire multisite.
Please oh please tell me there is a way to only tell the Members widgets and the BuddyPress All Members tab to only show members which are part of the given sub-site? Our clients are not going to like seeing members from other sites when they click "All Members" tab or view their widgets. It would really surprise me if this need has not come up before as I would think this would be a common problem for folks.
Any enlightenment you can give would be very much appreciated about now.