The number of Facebook's global users more than doubled in May from a year earlier, putting it ahead of MySpace in total unique visitors and showing which site has more momentum outside the U.S.

Facebook's unique visitors surged 162% to 124 million for the month, while MySpace's total eked up 5% to 114.6 million, according to comScore.

MySpace is still dominant in the U.S. market, with nearly 74 million monthly uniques versus 35.6 million for Facebook. But even here, Facebook is growing much faster, at 34% to 7%. 

The law of large numbers is responsible for part of MySpace's slower growth rate, and Facebook's U.S. growth is slowing as well.

But if it keeps posting growth rate that is many times that of MySpace, it's likely to pressure MySpace's U.S. lead at some point in the next two years.

And given the geographic breakdown of their audiences and relative growth rates, Facebook is likely to quickly pull well ahead of the News Corp. site.

Facebook gets just over 70% of its traffic outside the U.S., double the percentage for MySpace, so it's blistering growth outside the U.S. is going to keep its overall numbers surging.

The other social networks growing fast overseas were Hi5, up 74% to 50 million, and LinkedIn, which grew 78% off a much smaller base to 6.9 million.

Worldwide Unique Visitors To the Top Social Networks

Facebook—123.9 million, up 162%
MySpace—114.6 million, up 5%
Hi5—49.6 million, up 74%
Friendster—38.1 million, up 54%
Bebo—25.1 million, up 46%