The traffic numbers for social networking sites are in for September 2009 and according to HitWise, Facebook is the leading social networking both size and users. Facebook has 58.6% of all U.S. social networking visits for the month. That number represents a 194% growth increase over the market share Facebook held in September 2008.

EWeek reports that Facebook crossed the 300 million user mark last month and shows little signs of slowing down. Former category leader MySpace is now the second place social networking site, Tagged was number three with 2.4% of the market, and Twitter has 1.8% with MyYearbook grabbing 1% of the market.

Twitter may not have cracked the top five in the latest rankings, but for sheer growth, the micro blogging site is at the top. Twitter has grown its market share 1,170% from the paltry 0.15% of the market it held in September 2008. Twitter is also gearing up for some significant changes that will undoubtedly help it grow even faster. Twitter recently closed a new round of financing that netted an undisclosed amount of financing for the site.

Overall visits to social networking sites in the U.S. for September grew by 62% compared to 2008 and time spent on the sites increased by 20%. MySpace still holds the most time spent on the site by users crown with the highest average time spent on the site by users of 25 minutes and 56 seconds. Facebook however, had the largest time spent growth at 23%.

The most prolific age group using social networks continues to be 18 to 24 year olds with 32.3% of the visitors. Users 25 to 34 were 26.9% of the visitors. The fastest growing age group using Facebook is users older than 55 with an increase of 108% in September.