Twitter has 106m registered users

"[Twitter] has 106 million registered users who write 55 million posts a day.
Seventy-five percent of that traffic comes from outside Twitter, using third-party applications, like TweetDeck.
The site gets 180 million visits a month."
Source: Figures revealed by Biz Stone at the Chirp confernce, reported by New York Times, 14th April 2010