In March of 2008, Twitter was estimated to have one million active users, according to Michael Arrington of TechCrunch. But amazingly, over the course of the following year, Twitter was able to expand its user base by 32-37 times, according to which news source you trust for your data.
On July 30th 2009, the Philadelphia Enquirer reported that Twitter had "increased its user base by 37-fold to more than 32 million users worldwide, 18-20 million of whom reside in the United States".
Now, the naysayers are fond of pointing out that Twitter's attrition rate is quite large, with 60% of new users disappearing after signing up for the service. But if the Philadelphia Enquirer's estimation is correct that "only 40 percent of first-timers become habitual visitors", then the remaining 12.8 million users still make a viable and vibrant Twitter community.
Each individual user inside the Twitter community is capable of building his or her own community of Followers.
As Ashton Kutcher has taught us, building a Follower list is fairly straight forward - when people are interested in you and what you have to say, they will follow you on Twitter to see what you have to say.
To date, nearly 3 million Twitter users have decided that they care enough about Kutcher's life to follow his personal tweets. Of course, I am sure it helps that he occasionally posts pictures of his wife, Demi Moore, in his tweets. Not only did Kutcher share a shot of his Demi's derriere, he also proved that rich people have ugly furniture too.