It’s not news that Twitter is popular. But new numbers from comScore (NSDQ: SCOR) quantify just how astonishing the service’s growth has been, and reinforce why both Microsoft (NSDQ: MSFT) and Google (NSDQ: GOOG) are reportedly vying for a partnership with the microblogging site. The number of visitors to the site in March more than doubled to 9.3 million, up from 4.3 million in February. comScore’s Andrew Lipsman suggests that mainstream media attention may be driving much of the growth.

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He notes, for instance, that Twitter users are more likely than the average American to visit major Internet news sites. That, of course, may be because Twitter is sending lots of traffic to those sites. But it also could be that Twitter users are getting exposed to the service via the news. Lipsman writes, “If you watched the news this past week, you might’ve heard that Newt Gingrich levied criticism of President Obama’s response to the Somali pirate standoff over Twitter. I mean, we’re talking about the highest levels of government here, and a microblogging site is being used as a top politician’s primary media outlet?”