Nielsen/NetRatings this week delivered its regular monthly list of top social-media sites, with Facebook and LinkedIn tabbed as the strongest gainers of users.

With traffic to social networks surging—but also sites regularly moving in and out of vogue—the list offers an snapshot of the top of the social heap.

Notably absent, though not surprisingly so, are carrier and mobile social networks, which have just begun to emerge. It remains to be seen whether social networks more associated with the telecom world than the Web can make a dent on the list, including Nokia’s Mosh network or pure-play mobile social networks such as BluePulse.

It’s possible cell phones could simply become yet another route into existing social networks—accomplished via carrier partnerships, a la MySpace mobile deals—or via mobile Web pages or apps that cut carriers out of the equation altogether.

MySpace and Facebook are the top two social networking sites accessed via mobile devices in the U.S., according to analyst firm M:Metrics. MySpace attracts 3.7 million U.S. users, while Facebook’s mobile audience is about 2 million. Total U.S. mobile social networking users number just 7.5 million, or 3.5% of all mobile subscribers, M:Metrics said.

Yet some analysts are forecasting a bright future for mobile social network. According to U.K.-based consultancy Juniper Research, “mobile operator revenues from user-generated content will increase tenfold over the next five years, rising from $576 million in 2007 to $5.74 billion by 2012,”—with social networking services accounting for 50 percent of that total. The report forecasts the number of active mobile social network subscribers accelerating from 14 million in 2007 to 600 million in 2012.

Overall, the top of the Nielsen/NetRatings social network list for August 2007 is: MySpace (60 million users); Facebook (19 million); Classmates Online (15 million); Windows Live Spaces (9 million); AOL Hometown (7 million); Reunion.com (5 million); LinkedIn (4 million); Club Penguin (4 million); Buzznet.com (4 million); AOL People Connection (4 million).