I try to update my twitter profile regularly, but I am certainly not one of those who publish every minute. I blurb once a week or so. I share my twitter address at the end of presentations. If colleagues find me, I let them follow me, etc. Lately, I started getting requests from folks who are in related fields (or not) who want to connect with me on twitter. Every morning, I find 2-3 updates in my email, informing me that I am followed by folks I've never met before. We don't know each other, but we tell each other if we're bored, running around over the weekend, or frustrated at a baseball game.
I'm thinking, because the universe of twitter is much smaller, it doesn't take that long for an active professional to reach the tipping point. You may not have to wait until 100 followers, 30-40 may just do.