December data released by Nielsen examines year-over-year trends in unique visitor counts and time spent on social networking sites in the United States and around the world, including Facebook and Myspace.  US time spent on social networks increased faster than the average global increase of 82%, with the average US user spending 143% more time on social nets than last year.

Facebook, not surprisingly, was accountable for much of the increased social network usage in the US, with a 200% year-over-year increase in average time spent. A look at the traffic chart below shows just how rapidly Facebook has gained unique visitors. In December 2008, MySpace had slightly more unique visitors than Facebook, at just under 60 million, according to the Nielsen data. By December 2009, Facebook was well past 100 million US unique visitors, while MySpace lost users.

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A trend chart from Compete.com tells the story more succinctly. December 2008 was MySpace’s last month as the leading social networking site in the US.

Still, MySpace has more than 50 million users in the US, which makes it a Top 5 website in the US, according to Experian Hitwise data as of January 23, 2010.  Fifty million users means there is still a vibrant and loyal community on MySpace, while Facebook has brought a whole new audience to social networking – 30, 40 and 50 somethings.

With the exception of Twitter, Facebook’s explosive growth has stolen thunder from smaller, more focused networks like Classmates and LinkedIn.  Both had decreasing unique visitor counts according the Nielsen report, while Twitter grew 579% year-over-year, to 18.1 million in December 2009. Month-over-month, however, unique visitors decreased 5% for Twitter, a continuing trend since October. It may be that Twitter usage is still growing in the US, but it takes complicated measurement methodologies to determine the truth.