As a networking tool however, it's been way more effective than Facebook, LinkedIn, or any of the others I've used, and I fall very much into the casual/occassional user camp.
It helps to follow and get to know them a bit first, too. On Twitter, you can engaging in mini conversations with just about anyone, and once you're familiar with someone you can tweet at or DM them without coming off like a cold-call. Basically, just be yourself and it works wonders.
That said, I still tend to shy away from a lot of online interactions (just look at my stunted comment history on here for example ;). Yet when I do put myself out there, I tend to be surprised at the positive response.