While there were approximately 6 million Twitter users in the U.S. in 2008 -- or 3.8 percent of all Internet users -- eMarketer estimates that number will jump to 18.1 million in 2010, representing 10.8 percent of Internet users.
These estimates include adult users and companies that access Twitter at least once per month via any available platform, including the Twitter.com Web site, mobile devices and desktop applications.
Taking a look behind the data, the number of Twitter users is considerably greater than the number of visitors to Twitter.com as a result of the multiple access points for the service such as mobile units and desktop apps.
If media attention is any indication, Twitter has exploded into an all-out phenomenon.
Celebrities, politicians, entrepreneurs, business leaders and everyday users are flocking to the service en masse, generating a frenzy of activity and attention. Oprah Winfrey's move last week to join Twitter and Ashton Kutcher's well-publicized drive to amass 1 million followers have also fueled the fire.