A report released by Hitwise yesterday revealed that MySpace received 72 percent of the United States’ overall Web traffic from the 10 largest social networks in December 2007. But Facebook, which came in second with 16 percent, remains the fastest-growing social network.

The report measured the market share of social networking sites by calculating how many “visits” they received in December 2007 compared to the year before. A visit is defined as Web traffic directed at a domain or category of a social networking site.

Falling into third and fourth were Bebo and BlackPlanet, which both received around 1 percent.

The report painted a mixed picture for MySpace. In December 2007, about 95 percent of the site’s visitors were return users—people who had visited the site at least one other time in the same 30-day period—indicating a loyalty among its users. In addition, MySpace saw users spend an average of 29 minutes on the site, while Facebook trailed at 19 minutes. The one caveat: That was a nearly 10-minute increase for Facebook, while MySpace (which had 30 minutes last year) saw a slight downturn.

Nonetheless, both sites can use those numbers to help generate revenue from advertising.

“It’s a sign that [users] are coming back and interacting as much as possible,” says Heather Dougherty, Hitwise’s director of research.

“If you’re an advertiser looking to place media on the site, that’s very attractive.”

But MySpace also saw its growth rate shrink by 8 percent, paling in comparison to Facebook, which increased by 50 percent over last year. Jonathan Yarmis, an analyst with AMR Research, says those numbers indicate that Facebook has been able to evolve from a collegiate and high school phenomenon to engage a broader, older audience.

“The growth rate says it all,” Yarmis says. “By opening the platform, Facebook has seen a ton of growth from people who are not in the 20-something demographic. It’s to the eternal consternation of my son, who is 16, that I, at 52, have more friends than he does [on Facebook].”

What wasn’t immediately clear from the report was how many users entertain accounts in both Facebook and MySpace. Hitwise’s Dougherty couldn’t specify how many users had an account at both, but she did say that 7 percent of visitors to MySpace proceeded to visit Facebook immediately after. And 5 percent of Facebook users went to MySpace immediately after.