In the United States, Facebook grew by slightly more than five percent between July and August, according to web measurement firm comScore, from 87.7 million monthly unique users in July to 92.2 million last month. This is the second month that comScore has put Facebook in the top five, after Google, Yahoo, Microsoft and AOL — all of which have more than 100 million users.

Meanwhile, Facebook itself reported 81.2 million monthly active users at the end of August, in its advertising tool. ComScore and Facebook use different measurement methodologies, although the firm tends to be closer to Facebook’s own numbers than other third-parties.

ComScore also reported that Facebook, measured as an ad network, was viewed at least once by 46.7 percent of the total US Internet population in August — 33rd place, well behind various sorts of media conglomerates and other networks.

What does the firm report about Facebook’s competitors? Fox Interactive Media, which includes MySpace, dropped from 80.9 million monthly actives in July to 77.5 in August. And, surprisingly, Twitter also dropped. It had 21.2 million uniques in July but only 20.8 million last month.