Works on the same filters as the @atusername mention filter (see Extra Configuration if you want to enable this on blog/comments activity) – this will convert anything with a leading #
Warning: This plugin converts #hashtags prior to database insert/update. Uninstalling this plugin will not remove #hashtags links from the activity content.
The author says: “This plugin will not be updated for future versions of BuddyPress (1.3)”
BuddyPress Edit Activity Stream plugin 0.3.0 or greater
BuddyPress Activity Stream Ajax Notifier plugin
Download and info: http://wordpress.org/extend/plugins/buddypress-activity-stream-hashtags/