For STC communities, I think Twitter can be used to post those brief messages or notices about articles or events that are not (yet) blog-worthy. They can also be reminders of upcoming events.

For technical communication in particular, I have read about people using Twitter to ask for ideas and suggestions from their followers. For example, you are pondering an issue. By posting your question, you may receive references to articles and books that you weren't aware of, or you may receive just the suggestion you need to continue with renewed energy for your project. Not everyone has to answer, but just having a few people who answer can provide a few answers that you might not have found on your own.