The mobile media measurement company, in its first report from a study of social networking via mobile phones, found that although some 7.5 million Americans accessed mobile social networking sites last month, they accounted for only 3.5% of all mobile subscribers.

But according to Mark Donovan, senior analyst, M:Metrics, user adoption is set to grow, because "for an increasingly wide audience, social networking sites have become an indispensable part of the digital lifestyle."

Indeed, sites like MySpace and Facebook have forged on-deck agreements with mobile service providers such as Helio, Verizon and AT&T, with the goal of increasing audience penetration--and analysts predict the demographics of mobile social network visitors will mimic those in the online space, expanding beyond teens and early adopters.

"Nearly every online social networking site has added the ability to connect to these communities with a mobile phone," said Donovan. "With the mobile phone playing a central role in people's social lives, it's only natural that social networking sites are working to bridge the gap between the online and mobile worlds."

Compared to Western Europe, however, U.S. mobile subscribers are most apt to visit social networking sites on their handsets. M:Metrics studied mobile social network use in five countries (the U.K., France, Germany, Italy and Spain, in addition to the U.S.)--and while none topped the U.S. in use, Italians clearly had the social networking bug--as some 1.3 million users (or 2.8% of the subscriber population) visited a site like Bebo or MSN/Windows Live Spaces last month.

The U.K. came in second with 1.1 million (2.5%)--followed by Spain, with 751,000 (2.3%), Germany (1.9%) and France (1.7%). In the U.K., MySpace garnered the most mobile users, but MSN was the social networking forum of choice for users in the other countries.

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