The September numbers from market watcher comScore show Facebook Inc.'s users in the U.S. grew by 4 percent to 95.5 million while Twitter Inc.'s numbers flattened out at about 20.8 million.
MySpace is still in second place behind Facebook, but scores of users have begun walking away.
Only a year ago, MySpace held a commanding lead of 66.8 percent among the 155 social networking sites studied. But since then, U.S. users' visits to MySpace have sky-dived by 55 percent to a total of just 30.2 percent, leaving Facebook in the top position.
The gap between Facebook and San Francisco micro-blogging service Twitter has grown by about 50 percent from April to September. Facebook's 50 million user lead has grown to about 75 million over the summer, according to the comScore report.
Overall, U.S. visits to social networking Web sites rose 62 percent from September 2008 to September 2009, according to Hitwise, which tracks 155 such sites.