­Time spent using mobile devices for activities such as internet and app use, gaming, music and others has more than doubled in the past two years among USA consumers.

This year, the amount of time consumers spent using mobile devices -- excluding talk time -- will grow 51.9% to an average 82 minutes per day, up from just 34 minutes in 2010, eMarketer estimates.

Mobile growth comes as time spent online, which includes desktop and laptop computers, internet-connected TVs and other nonmobile connected devices, sees its own growth rates decline -- though it is still growing. This year, eMarketer estimates time spent online will grow just 3.6% to an average 173 minutes per day, compared to 7.7% growth in 2011 to 167 minutes per day.

Since online already accounts for an average of nearly three hours of time spent with media each day, double-digit growth rates are unlikely even as consumers continue to do more activities and spend more time with their desktop and laptop computers.

Mobile, by contrast, is growing quickly from a small base -- and growth in time spent is also being boosted by fast uptake of smartphones and tablets, which have still penetrated only a minority of all consumers. As more continue to acquire these devices, and current owners shift more of their digital activities to mobile and portable devices, mobile is grabbing an ever-greater share of consumer time with all media -- potentially at the expense of faster online growth.

This shift has been problematic for some advertising publishers who have not necessarily seen ad dollars follow suit. Even on the desktop web, ad dollars are still playing catch-up to time spent with the medium -- and that game is even further behind on mobile, which will see just 1.6% of US ad spending this year even as adults spend an average of 11.7% of their media time with their phones and tablets.

While mobile advertising is expected to grow rapidly in the next few years -- eMarketer estimated in September that overall US mobile advertising spending will reach $2.61 billion this year before rising to nearly $12 billion in 2016 -- there are some significant barriers that both marketers and ad publishers will have to overcome before the mobile ad spending will achieve parity with the share of time spent by consumers on mobile devices.

In the case of search advertising, for example, the lower likelihood of consumers making purchases on mobile devices -- which is reflected by the small footprint that mobile commerce holds in the overall ecommerce market -- has forced companies like Google to charge lower rates for mobile search ads.

In the case of display advertising, small screen sizes have limited the ability of display ad publishers like Facebook to serve as many impressions per page view as they might to desktop users.

As outlined in eMarketer's mobile advertising spending forecast from September, both advertisers and publishers are still working to develop the infrastructure necessary to support larger mobile ad buys.

By contrast, consumers spend an average of 24.8% of their media time online with desktop and other nonmobile internet devices, while spending on advertisements on those same devices accounts for 20.9% of total media ad spending in the US.

Despite continued concerns about cord-cutting, time spent watching television continues to increase -- growing to an average 278 minutes per day in 2012, up from 274 minutes in 2011 -- though TV's share of the total pie will decrease marginally, by less than 1 percentage point, over the same period.

The largest shift downward continues to come from the print media sector. Time spent with print media will drop to an average 38 minutes per day this year, eMarketer estimates, down from an average 44 minutes per day in 2011. Newspapers will see a drop to an average 22 minutes per day this year, while time spent with print magazines will fall to 16 minutes per day.

Time spent listening to radio will remain above mobile, though its share of total time spent with media continues to gradually decline.