This just in from web analytics firm Hitwise. Social network MySpace continues to see the most U.S. traffic — 72.73 percent share of the social networking market — but that’s a slight drop from its April traffic and a five percent drop from the previous May.

Meanwhile, the only other significantly large social network in the U.S., Facebook, grew 42 percent this May over last, seemingly at MySpace’s expense. See graph below for more. As I recently argued, Facebook’s advantage appears to be that it captures real-world relationships in a way that its rivals don’t, at least not as much. This may account for its increasing market share, although proving such causal relationships has yet to be done.

Aside: MyYearbook, a high school social network, keeps growing fast, even though it’s far, far smaller than either of its rivals.

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