I have a network of blog, everyone with it's own installation of wordpress. Now I want to add social features to the sites but I want to unify all the activity of the blogs in one place. I would prefer to have every blog in it's own installation. Is there a way to install buddypress in one site and collect there the activity of all blogs? and have the user accounts synced across the network?
Why would you prefer individual installations? You can enable/disable themes on a per-blog basis and do the same for plugins using Multisite Plugin Manager.