I've been crawling Twitter user data for awhile, and I can tell you that the # of ACTIVE twitter users is somewhere between 15 million and 25 million users.<p>I notice this because in the beginning, I'd be crawling a ton of users a day, but have reached a peak in the past few weeks as I've crawled more and more (now I only crawl maybe 10,000 new users a day that weren't crawled already).