Facebook is leaping ahead of MySpace to turn into the most popular social networking site in the U.S. Yet although Facebook nailed down 58.6 percent of all U.S visits to social networking sites in September -- for an increase of 194 percent -- use of Twitter surged even more astoundingly, according to Experian Hitwise.

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 plummeted by 55 percent to a total of just 30.2 percent, leaving Facebook the new winner in the social networking sweepstakes, say statistics released by the number cruncher last week.

Other networking sites, including Twitter, still lag way behind the two lead runners. But Twitter, a site that got only 0.15 percent of all U.S. visits in September of last year, ended in fourth place overall last month with a 1.8 percent share of U.S. visits and a stellar growth rate of 1170 percent.

Tagged came in third last month, with 2.3 percent of the pie and a year-over-year percent rise of 47 percent. Meanwhile, myYearbook dropped 40 percent, finishing last month in fifth place among the 155 at 1.0 percent.

The new stats from Experian Hitwise corroborate earlier signs that MySpace is slipping against Facebook which showed up in a June study by ComScore.

Apparently, the exit of co-founder Chris DeWolfe from the company in April hasn't done much in terms of helping MySpace to either reinvent itself or regain market share.

At the same time, Facebook has been adding new features such as Facebook Connect, to let users easily translate other sites and apps across the Web in other human languages.

Facebook's booming popularity seems to be bearing financial fruit, too. Also in September, Facebook grabbed its 300 millionth user while claiming that it had become "cash flow positive" months ahead of expectations.