More than 72.2 million Americans, or nearly one-third of the country, accessed Facebook, LinkedIn (NASDAQ:LNKD), Twitter, or some other social network or blog from a mobile device in August, up 37 percent from the same time last year.

Nearly 40 million of those U.S. mobile users access these sites almost every day, according to new research from comScore. Smartphone users proved to be the heaviest social media users, with 3 in 5 of those users using social media software every month.

Facebook, which claims it has over 200 million mobile users, enjoyed more than 57 million mobile users in August, up 50 percent from the previous year. Twitter and LinkedIn have far fewer mobile users, according to comScore. Twitter's mobile audience rose 75 percent to 13.4 million people, while LinkedIn's audience grew 69 percent to 5.5 million users.

In culling its numbers, comScore counted users who accessed those social networks from a Web browser, social app or both. For example, users access Twitter from hundreds of third-party apps.

Indeed, consumers are taking two main roads to the social media software: the Web browser and applications. comScore found that while more Americans accessed Facebook, LinkedIn and Twitter via their mobile browser, the social networking app audience grew five times faster in the past year.

Specifically, the mobile browsing social networking audience grew 24 percent to 42.3 million users in the past year, while the mobile social networking app audience leapt 126 percent to 38.5 million.

The trend-which validates Forrester Research's thesis statement for the so-called "App Economy," where software access is done more via individual apps than the Web-underscores the value proposition behind mobile app stores.

Apple's App Store and Google's Android Market are proving to be popular places for application developers to make some money from paid apps, or simply taking a cut through advertising served with their apps. That's Google's big lure for app makers.

What are U.S. users doing with social media on their devices? For August, 80.3 percent of people read posts from people they know personally, while 69.5 percent posted status updates while on their mobile device.

More than half of the mobile social media users read a post from an organization, brand or event while on their phone or tablet. Also, one in three mobile social networkers received a coupon, offer or deal from their smartphone or tablet, while 27.7 percent admitted to clicking on an ad while on a social networking site.

comScore analyst Mark Donovan noted that these engagement rates should be a wake-up call for advertisers and brand marketers to target users with location-based services. For example, pinging users with a coupon or offer for a favorite restaurant, coffee shop or theater performance can help lure more users.

Meanwhile, Facebook, Twitter, LinkedIn and Groupon are competing harder for users' attention. Facebook claims to have over 350 million total mobile users, so it will be interesting to see what comScore's count is for users of that social network for its next quarterly count in December.