Ning, social network builder

Twitter.com, Tagged.com and Ning racked up 343%, 330% and 251% in year-over-year audience growth, respectively — making them the fastest-growing US online social networking sites in September, according to (pdf) rankings from Nielsen Online, MarketingCharts reports.

MySpace.com, with 59.4 million unique visitors in September, topped the month's list of US social networking sites and grew one percent over September 2007. It was followed by Facebook, with 39.0 million unique visitors, and a growth rate of 116%.

About the research: The social-network site rankings are custom lists compiled by the Nielsen Online PR team with the help of Nielsen media analysts. While these lists are not exhaustive, they provide directional guidance as to the significant players in each space. Nielsen Online ranks social networking sites by unique audience and rounds to the nearest thousand.