Twitter’s march towards world domination continues apace. This morning comScore released its global numbers for March, 2009. Worldwide visitors to Twitter.com increased 95 percent in the month of March from 9.8 million to 19.1 million, according to its estimates. This compares to 9.3 million visitors in the U.S. alone.These numbers only count visitors to Twitter’s Website, which is not the same as active users and also does not include people who interact with Twitter via desktop or mobile clients (a large portion of users). But the comScore numbers provide a good proxy for Twitter’s overall growth, which was helped recently by Ashton Kutcher’s race with CNN to one million followers, and Oprah’s subsequent adoption of the service.If Twitter can keep this rate of growth up, it should cross 50 million visitors by summer.