M:Metrics today released for the first time its measurement of mobile social networking, announcing that 12.3 million consumers in the United States and Western Europe reported accessing a social networking site with their mobile device in the month of June.

The American audience for mobile social networking sites was the largest, with 7.5 million, or 3.5 percent, of mobile subscribers. Italy follows, with 1.3 million or 2.8 percent, then the UK with 1.1 million, or 2.5 percent, Spain with 751,000 (2.3 percent), Germany (1.9 percent) and France (1.7 percent). MySpace garnered the most mobile users in the United States and United Kingdom whereas MSN was the forum of choice for mobile Web 2.0 users in the other geographies surveyed.

"For an increasingly wide audience, social networking sites have become an indispensable part of the digital lifestyle," said Mark Donovan, senior analyst, M:Metrics.

"Nearly every online social networking site has added the ability to connect to these communities with a mobile phone, allowing people to access profiles and share content while they’re on the go. 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."

MySpace and Facebook are the top two social networking sites accessed via mobile in both the U.S. and UK. MySpace attracts 3.7 million U.S. and 440,000 UK mobile users. In America, Facebook’s mobile audience is about 2 million, and in Britain, about 307,000. Rounding out the top three is YouTube in the U.S., with 901,000 mobile visitors and Bebo in the UK, with 288,000.

M:Metrics also tracks the availability of content across all operator portals and found that in June, the top three U.S. mobile social networking sites had strong distribution, with MySpace appearing on the decks of : Amp’d, AT&T, Helio and Nextel. Facebook was accessible on the Sprint, AT&T, Virgin and Amp’d decks and YouTube on the Verizon deck*. In this UK, MySpace has distribution on Vodafone, Bebo is on the 3 deck, but number-two MSN Live Spaces is offered nowhere on-portal.

Those under 25 are the most active users of mobile social networking sites across all geographies. In France, Germany, Italy and Spain, the age demographic with the largest percentage of use is 13-17-year-olds, whereas college-aged consumers (18-24) are the most avid users in the United States and the United Kingdom.