• by Laurie Sullivan , Staff Writer @lauriesullivan, February 11, 2013

Mobile adoption and online ad spend are soaring.

Online advertising should reach $50.8 billion in the United States by 2015 -- up from $36 billion this year. Search will contribute nearly $19 billion, and social media, $8.4 billion -- up from $15 billion and $4.6 billion this year, all according to the ZenithOptimedia New Media Forecasts report.

Through the ads, marketers will attempt to connect with an estimated 268,283 million Internet users by 2015 in the U.S. -- up from 257,362 million this year, estimates eMarketer, MRI Fall 2011, and Zenith Advertising Expenditure Forecast.

Individuals going online -- most to connect with others through email and social networks -- will access content through a variety of devices. Expect about 26.1% to own tablets or ereaders, and 58.6% to own smartphones by 2015 -- up from 19.4% and 42.4%, respectively, in 2013.

The study identifies 19 digital markets across the world, ranked according to size and percentage of total ad spend, and examines how consumers in the marketers use digital media; how Internet advertising, social media and e-commerce continue to develop; and the pace of new media adoption.

Adults ages 18 to 34 use the Internet for an estimated 1.85 hours daily on average, equivalent to nearly 20% of their daily media consumption. While it's the highest use among the age groups, online still falls behind television and radio consumption. Print media emails are less important to this age group, with newspapers averaging less than 30 minutes daily. They use if for work and play, such as to pay bills and shop.

Online buyers represent 70.3% of Internet users. Online shoppers number 87.5%, although growth should slow to an estimated rate of 8.8% during the next five years, totaling 79.1% by 2016. In fact, $1.115 billion of U.S. offline sales -- 41% of total retail sales -- in 2011 were influenced by Web content, according to the study, which cites eMarketer.

The average adult age 55 years and older flips the numbers that marketers identify with adults ages 10 to 34. This age group spends 7.49 hours watching television daily and less than one hour per day on the Internet. They also read newspapers more often, with 0.56 hours of reading per day.