You are interested in a subject, but you don't know what hashtag people are using. No problem. Use Hashtagify.me. Say you were interested in the "flipped classroom." Type that term into Hashtagify.me and see what happens.
Libraries are increasingly getting hip to using Twitter as a tool, with many offering the service as a point of contact with librarians. But Twitter is an excellent tool for librarian learning as well, offering lots of great opportunities for discussion. Many Twitter chats exist for the literary and library world, and we've discovered 20 of the absolute best for librarians to check out.