The US accounted for over a third of global ad spend in 2014

"The global advertising market surpassed a half-trillion dollars in 2014 totaling $525 billion with the top 10 countries accounting for nearly three-quarters of global advertising spend, according to a report from Strategy Analytics entitled Global Advertising Market Forecast Outlook.
The US alone accounted for over a third of global ad spend in 2014, 3.8 times higher than the second largest market, China, and over 16 times larger than Canada, the 10th largest market. Meanwhile China surpassed Japan in 2013 to become the second largest advertising market and will surpass Germany (4th largest market) and the UK (5th largest market) markets combined in 2015.
The US also has by far the largest TV advertising market. In 2014 it was nearly 5 times larger than China, the second largest market."

Source:  Advanced Television, 4th March 2015
Note - Press release from Strategy Analytics here

Image showing top 10 markets below