The social network added about 4 million unique visitors in March to reach 61.2 million and close within 9 million of MySpace, the top social network in the U.S. But while MySpace’s audience of 70.2 million has shrunk by 4% in the last year, Facebook’s has jumped 72%.

At the current rate of growth, Facebook is poised to overtake its social networking rival in the U.S. in the next few months. The company already surpassed MySpace last year as the most-trafficked social network globally. Facebook this month announced that it crossed the 200 million mark in active users worldwide, while Facebook has an estimated 130 million.

At stake in the U.S. are more than bragging rights, however, since the domestic market is where most of the ad dollars that Facebook and MySpace compete for come from. Facebook isn’t the only social property moving up. ComScore reported Wednesday that Twitter’s audience nearly tripled to 9.3 million in the last month as broader media coverage of the micro-blogging phenomenon helped propel growth.

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