Boom! Twitter More Than Doubles Unique U.S. Visitors To 9.3 Million In March

If it seems like Twitter is growing faster and faster each day, that is because it is. ComScore has released its March numbers for the U.S., and it estimates that unique visitors to grew 131 percent between February and March to 9.3 million visitors. No wonder Twitter is more popular than Britney.

Not only did Twitter more than double the number of people that go to its site in a single month, but it accelerated its growth from the 55 percent rate it experienced in February. These numbers do not include international visitors, nor do they include all the usage on desktop and mobile clients, which is significant in Twitter’s case. But it is a useful proxy.

So to just to give a sense of the type of growth Twitter is going through, here is the month-to-month growth in U.S. unique visitors so far this year:

March, 2009: 131%
February, 2009: 55%
January, 2009: 33%

In February, comScore estimated that had worldwide 9.8 million visitors worldwide and 4 million U.S. visitors. If that 41 percent ratio of U.S. visitors to total worldwide visitors still holds (in January it was about the same), it would mean that attracted more than 20 million unique visitors worldwide. ComScore releases international figures later in the month.