Facebook has eclipsed rival social network MySpace in terms of unique visitors in the United States, according to online metrics firm comScore.

In May, Facebook logged 70.29 million U.S. visitors, edging out MySpace, which saw 70.24 million uniques.

In April, MySpace beat out Facebook by about 3.5 million visitors. But over the past year, Facebook's surging popularity has been moving quickly to close the gap with MySpace, which has seen its traffic actually post sequential declines in several given months.

In May of 2008, MySpace enjoyed a substantial domestic lead, with 73.7 million unique visitors, compared to the 35.6 million Americans who visited Facebook.

Globally, it's been a different story. The lion's share of Facebook's members live overseas, and the site rocketed past MySpace in global traffic last August.

It hasn't looked back since. In April, the most recent month that comScore's international data is available, Facebook enjoyed 307.1 million visitors worldwide, compared to the 123.3 million who visited MySpace.

Much has been made of the two companies being on seemingly different trajectories. Facebook, which recently moved into a spacious new office campus in Palo Alto, has been hiring briskly and is now projecting that it will be cash-flow positive by 2010.

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The company recently ushering out co-founder and CEO Chris DeWolfe, and swiftly replacing him with Facebook veteran Owen Van Natta.

By comScore's measure, Twitter saw the third-most U.S. visitors in May, an eye-popping 2,681 percent increase from the same month last year.