GoogleAdBusiness This chart shows Google advertising revenue compare to total ad spend in the largest ad markets in 2018. Photo: Statista/IBT

Despite the many activities that Google's parent company Alphabet is engaged in, the lion’s share of its revenue still stems from the core Google segment and its advertising business in particular, which has made Google the largest ad-selling company in the world by far. 

As our chart illustrates, Google’s ad revenue amounted to $116.3 billion last year, surpassing total advertising expenditure in all countries except for the United States. In 2018, Google raked in 32 percent of net global digital ad revenue and 38 percent of digital U.S. ad revenue. Adweek projects that digital ad spending will surpass offline ad spending in the U.S. in 2019, bringing even more opportunities to digital ad giants Google, Facebook and also Amazon.

While digital ad sales are dominated by the two big players Google and Facebook, Amazon has recently been trying to break up the duopoly. With Google's share at 38 percent, Facebook is earning roughly another 20 percent of U.S. digital ad market revenues. While Facebook's ad revenue is expected to keep growing with the help of Instagram, Google's is expected to decrease slightly in the light of the increased competition.