• by Mark Walsh  @markfwal, March 3, 2010

Facebook and Twitter both saw triple-digit traffic growth on the mobile Web even as MySpace lost ground to its social networking rivals among cell phone users in the last year. Facebook's U.S. audience on the mobile Web more than doubled to 25.1 million in January 2010 compared to a year ago, while Twitter's grew fourfold to 4.7 million, according to data released Wednesday by comScore.

By contrast, MySpace's mobile Web audience declined 7% to 11.4 million during the same period. (The figures don't reflect access to the social networking services by the nearly 6 million cell subscribers who do so exclusively via mobile applications, said comScore.) Overall, 11.1% of U.S. mobile users went to a social networking site in January, up from 4.6% a year ago.

Much of that growth has been driven by the proliferation of smartphone owners, who tend to be overrepresented on the mobile Web. Nearly one-third (30.8%) of smartphone users visited social sites on their devices, up from 22.5% last year. Only about 7% of regular cell phone users went to a social site.

"Social networking remains one of the most popular and fastest-growing behaviors on both the PC-based Internet and the mobile Web," said Mark Donovan, comScore senior vice president of mobile, in a statement. That's especially true of Facebook, which recently surpassed 400 million active Internet users worldwide and 100 mobile users worldwide.

Starting from a smaller base, Twitter has outpaced Facebook's growth in the last year, but during the second half of 2009 the microblogging service's Web traffic leveled off at about 20 million monthly visitors. Andrew Lipsman, senior director of industry analysis at comScore, said online and mobile traffic have shown similar patterns in the last year, although mobile "continued to climb, where online was actually flattening for awhile." That makes sense, since the bulk of Twitter activity takes place on mobile devices.

For MySpace, its shrinking mobile Web audience reflects the company's wider struggles to maintain share in the social networking space. Facebook bypassed the MySpace mobile browers audience a year ago -- three months before it did the same on the desktop Internet, according to comScore.

TechCrunch reported Tuesday that John Faith, formerly general manager and vice president of MySpace Mobile, left the company in January. Replacing Faith and taking charge of the mobile division is Nat Brown, whom the tech blog described as "once one of Microsoft's foremost technical minds." Reviving MySpace's mobile fortunes will make good use of brain power.