BuddyPress Hashtags for BuddyPress and bbPress. It allows adding hashtag links to BuddyPress activity and bbPress topics. Hashtags turn into links that are used to search items based on topics.
BuddyPress Hashtags offers dedicated widgets for BuddyPress Activities and bbPress Forums, you can display them at desired sidebars. It will have the most used hashtags throughout the entire community. It also gives you sorting methods and limit to display a certain number of hashtags.