• by Mark Walsh  @markfwal, October 20, 2011

Among top social networking services, Twitter had the fastest-growing U.S. mobile audience for the three months ending in August, up 75% to 13.4 million. LinkedIn was a close second, growing 69% to 5.5 million -- followed by Facebook, up 50% to 57.3 million, according to a new comScore report. In addition to the biggest U.S. mobile audience, Facebook also has the largest worldwide, at more than 350 million.

Twitter's mobile growth should get another boost through the service's integration into the latest version of Apple's mobile operating system, iOS 5. The new iPhone 4S running the upgraded software sold a record 4 million units in its opening weekend.

The comScore study found the social-networking population on mobile devices had increased 37% to 72 million in the last year ending in August. That’s equal to about 31% of all U.S. mobile users ages 13 and over.

Even more significant: the number of people who access a social networking site or blog on a daily basis has grown 58% in the last year to 40 million, or about 17% of mobile customers.

When it comes to how people access social sites, apps have nearly caught up with the mobile Web in the last year. The number of users who turn to apps has more than doubled (up 126%) to 38.5 million, while the Web-based social networking audience increased 24% to 42 million. Adoption of HTML5 may fuel the mobile Web’s growth, but the pendulum is swinging toward apps as the medium of choice for mobile social properties.

What are people actually doing on mobile social sites? More than half (53%) are reading posts from marketers and other organizations or about events. That’s an even larger proportion than those who read updates about celebrities and public figures (45%). About one-third received a coupon or offer and more than a quarter (28%) clicked on an ad over the three-month period.

The most popular activities were reading posts from friends (80%) and posting status updates (70%). More than half (53%) followed a posted link to a Web site.

“Advertisers and marketers should take note -– mobile users are not only engaging with their friends through social networking, but a majority are also interacting with brands in these social media environments,” stated Mark Donovan, comScore senior vice president for mobile. “Knowing that fans and followers engage with branded content on mobile devices opens the door to a world of opportunity for location-based services.” 

In that vein, Google yesterday announced the launch of contextual ads for Android and iOS apps that run against local searches. “Apps that provide a search experience will also be able to show ads related to those searches. Users will receive ads relevant to what they're looking for, and our developer partners will have a new tool to monetize their great content and services,” explained the company in a blog post Thursday.

Google rival Facebook has yet to begin monetizing its large mobile audience, partly because of concerns about undermining the user experience before coming up with the right ad solution for handheld devices. However, Facebook has taken steps toward ramping up mobile ad efforts, including the acquisition of mobile ad start-up Rel8tion earlier this year.