Mobile apps are now the most common way for U.S. citizens to consume digital media, according to a report by comScore. Desktop PC usage is declining as mobile devices become capable of more complex tasks.

"It's clear that mobile usage is a tidal wave that's completely transformed the way we consume digital content, particularly in just the past year or two. And we know that as eyeballs move to mobile, ad dollars will eventually have to follow suit. But this transition will be bumpy if the ad measurement and monetization infrastructure develops too slowly," states a comScore release.

Apps on smartphones and tablets now account for 51 percent of time spent on digital devices. Last year mobile devices accounted for 50 percent of digital media use. However the latest data shows that has increased to 60 percent, with desktop use declining to 40 percent. Apps are continuing to be developed for more daily tasks, and some forms of media use have switched almost exlusively to mobile devices, according to the comScore blog.

Analysis of the leading content categories (based on those with at least 10 billion minutes of total engagement in May) shows which ones have seen the most pronounced shift to mobile. "Amazingly, but perhaps not altogether unexpectedly, a couple of important categories have shifted almost exclusively to mobile," comScore's report notes. "Digital radio, led by category leader Pandora, now generates more than 96% of its total engagement from mobile devices."

Photos share in that high engagement rate as well, with photos used on mobile 96 percent of the time with Flickr and Instagram leading the way. Desktop use accounts for only 4 percent. Not far behind are maps and instant messaging systems, which see 90 percent of their usage on mobile. And, as of the latest report, 86 percent of time spent playing games is through apps.

Oddly, mobile devices account for only 76 percent of telecommunications use. As smartphones are performing more functions previously exclusive to desktop computers, PCs are performing more functions previously exclusive to phones as well.

ComScore notes 73 percent of downloads take place on mobile devices. The limited bandwidth available through a network connection would suggest that consumers would be more likely to use a PC, but downloads of new apps may be a large factor here. Music is listened to on mobile 72 percent of the time, and 71 percent of social media use now takes place on smartphones and tablets. Directories and resources still show a strong PC presence, but even there mobile devices are taking over with 65 percent of total usage time.

Although social networking is not the most mobile-dominant category, it is largely responsible for the overall growth in mobile usage. Some 20 percent of all time spent on digital devices is using social media, and 70 percent of that is through mobile devices. Facebook's app alone is responsible for 18 percent of total time on mobile devices, while 24 percent of all mobile time is spent on Facebook.

The use of social media on mobile devices has grown by 55 percent over last year, accounting for 31 percent of all growth in digital media consumption. As apps like Facebook and Twitter continue to gain ground, consumers are turning toward tablets and smartphones. Although the devices are less powerful then PCs, social netwoking doesn't require the incredible processing speeds available to modern desktops.

Although games and visual-editing software take advantage of the increasing power level of PCs, the most common uses for digital devices don't require more processing power than a smartphone can provide. As smartphone and tablet technology continues to progress, we are likely to see the devices occupy an even greater share of the market, with ads and monetezation strategies to follow.