In the war of social media, Facebook has got a leg up on Twitter, according to the latest numbers measuring the popular sites.

In the month of July, Facebook attracted 87.7 million unique visitors in the U.S., which was 14 percent higher than in June 2009, according to estimates made by comScore. Twitter, on the other hand, only saw 21 million unique U.S. visitors to its Web site, a 6 percent rise compared to the month before. Or in other words, Facebook added about 10 million new visitors in the month of July versus roughly one million new visitors for Twitter.

These estimates, according to TechCrunch, are only for, and do not include mobile or desktop applications such as Tweetdeck, Seesmic, or Tweetie. On an annual basis, Twitter is still growing its audience much faster (2,614 percent) than Facebook (124 percent) because it is coming off such a smaller initial base of users and this was the year it entered hypergrowth.

But, that growth seems to have slowed since the end of April, at least in the U.S. because between April and July, Facebook grew 30 percent in unique U.S. visitors, while Twitter only grew 25 percent.