MySpace attracted 76.35% of all visitors to the 53 leading social networking sites, Hitwise said. Facebook was a distant second with 12.57%, followed by Bebo, 1.24%, and BlackPlanet, 0.87%. The remaining Web sites on the list accounted for 8.97% of U.S. visits.
"MySpace and Facebook remain the market leaders among social networking websites, retaining a loyal base of users," Heather Dougherty, director of research at Hitwise, said in a statement.
MySpace in December received 95% of its share of traffic from returning visitors, while the number for Facebook was 93%. MySpace visitors on average, however, spent about 30 seconds less time on the site than the same month a year ago. Facebook visitors, on the other hand, spent about 10 minutes longer on average.
Of the top five social networking sites, Bebo's visitors spent the most time on the site on average -- 30 minutes and 24 seconds.
Also in December, Facebook showed the biggest growth of the top four sites, increasing market share by 51% to 16.03%, Hitwise said. MySpace's share fell 8% to 72.32%. Bebo and BlackPanet increased their share by 10% and 8%, respectively.
U.S. traffic to all the social networking sites increased 4% year-to-year in December, with MyYearbook, which was number seven in the market, recording the biggest jump. The site increased it share in the last month of the year 407% to 0.73%.
Overall, the numbers showed that online social networks remained a viable channel for advertisers. "The continued popularity of social networking activities online will drive the interest of marketers seeking to target influential users to approve and advocate their products and services," Dougherty said.