One of our favorite parlor games here at TechCrunch is trying to guess when exactly Facebook will pass MySpace to become the No. 1 social network in the U.S. by unique visitors. Worldwide, Facebook took that crown long ago (in April, 2008). But in the U.S., MySpace has been more difficult to displace.

At the end of last year, according to comScore, the gap in unique U.S. visitors was just over 20 million in MySpace’s favor. At that time, we projected then-current growth rates for both services and calculated that Facebook would pass MySpace no later than January, 2010. It looks like that estimate might be way too conservative. Today, Facebook has narrowed that gap to 9.1 million unique U.S. visitors, and is now on track to pass MySpace in the U.S. within the next three months.

Based on comScore’s latest March data, 61.2 million individuals visited Facebook in the U.S. during the month, compared to 70.2 million for MySpace. Facebook grew by 3.8 million visitors in the month, up 6.7 percent. Meanwhile, MySpace is losing visitors. In March, it attracted 160,000 fewer U.S. visitors than in February, and a whopping 5.8 million less than in January, when it had 76 million U.S. visitors. MySpace is also languishing internationally (based on February numbers).

If MySpace stays flat or loses more momentum, Facebook should have no trouble catching up to it in the U.S by summer.