BuddyPress.org

First of all, this more than likely is not an issue with BuddyPress, but an issue with the Share This plugin.

You need to contact the developer of that plugin and ask them if their plugin works with BuddyPress. It does not mention that it does. Second, if you are running WPMU 2.8.1, then ask them when they plan to put out an upgrade for 2.8.1.