I'm not exactly sure when, but we recently passed 5000 followers.
Twitter is a nice place to share useful articles, engage with potential users, and build some brand awareness. It's a little hard to get noticed on Twitter and does take a lot of work, and it's a little hard to gauge if it's actually helping us. I think the number of followers is a vanity metric to be honest, but nonetheless we did it! I think something like leads generated from Twitter might be a better measuring stick, but not sure to be honest. I'd love to hear what other IHers think about Twitter. Is it useful to you?