Facebook has been growing by leaps and bounds internationally, but how is it doing in the U.S? This question matters, because Facebook hopes to make billions off of advertising one day, and the U.S. is the single largest advertising market in the world. The problem for Facebook is that MySpace seems to have captured the market. Or has it?

Facebook grew by 58 percent from December 2007 to December 2008, according to market research firm Hitwise. The social network now gets 1.65 percent of all U.S. web visits. Meanwhile, MySpace has fallen 21 percent over the same period, from 4.72 percent to 3.71 percent of the market. Other major social networks, like Bebo, hi5, Friendster and Orkut, hardly register. Indeed, the only other site seeing growth is teen social network MyYearbook. It grew 115 percent, from 0.05 percent to 0.10 percent of the market — a rounding error in comparison to Facebook and MySpace.

The table below, of comScore data from November, shows MySpace still growing in terms of absolute unique visitors. The problem is that the rest of the web has been growing — and Facebook is getting itself a bigger piece of that pie, while MySpace is getting a relatively smaller piece.

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