, 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., 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%.