Advertisers are spending more on social media than they are on press for the first time, according to Zenith's Advertising Expenditure Forecasts report.
While spend on social media will be up 20% to $84bn (£68.3bn) by the end of 2019, press is set to fall 6% to $69bn.
The report, which forecasts a 4.4% growth in global adspend to $640bn – marking a 0.2% fall on its previous estimate – found that the US ad market is the world's most buoyant, with North America expected to see 5.5% growth, while western and central Europe will grow 2%.
Social media will command a 13% share of global adspend, behind TV at 29% and paid search at 17%. Growth in social media is expected to slow but remain in double digits for three years, dropping to 17% in 2020 and 13% in 2021, by which point it will account for 16% of global adspend.
Agencies are unlikely to be reaping much of the harvest, however; North America's growth has been driven by the rise of new small and medium-sized companies using Facebook and Google to advertise.
Matt James, Zenith's global brand president, said: "Social media advertising gives brands the opportunity to drive growth by using automated tools to optimise their campaigns for key business objectives.
"By using first-party data from their own websites to identify potential customers on social media, brands can convert consumers who are already on the path to purchase and target lookalike audiences more effectively."
Meanwhile, paid search advertising will exceed $100bn for the first time.
Europe has experienced growth since 2014 of between 3% to 5% a year and overall Zenith expects the region to continue to grow at the low end of this range into 2021.
However, Zenith has again downgraded its 2019 forecasts for the region's two biggest markets, Germany and the UK, due to economic decline in the second quarter. The agency has reduced its growth forecast for the UK from 3.9% to 3.3% this year, its slowest rate of growth since 2011, while Germany's decline has increased from 0.4% to 0.6%.
Zenith's last forecast, published in June, found that more than half (52%) of global advertising expenditure will be spent on internet advertising for the first time in 2021.