how many users on twitter?

How many LinkedIn users: http://press.linkedin.com/faq
How many Facebook users: http://www.facebook.com/press/info.ph...

With Twitter's popularity, I'm surprised that the company doesn't publish any stats (that I could easily find) of how many users there are?

The only thing I could easily find is...
http://siteanalytics.compete.com/twit...
...but that's just unique visitors, not user accounts or active users.

Anyone else ever asked this same question?