Jeff Bezos, Chairman and CEO of Amazon, speaks at the George W. Bush Presidential Center's Forum on Leadership in Dallas, Texas, U.S., April 20, 2018.

Amazon's advertising business will take market share from Google in 2019, according to new estimates by research firm eMarketer.

Amazon is projected to claim 8.8 percent of U.S. digital ad spending in 2019, up from 6.8 percent in 2018, and could reach 10 percent by next year, according to the study.

Market leader Google, by contrast, is expected to lose 1 point of market share, dropping from 38.2 percent to 37.2 percent. Facebook is expected to pull 22.1 percent of digital ad spending in 2019, just barely up from 21.8 percent last year.

The numbers are a boon for Amazon's growing advertising segment, which offers a third stream of revenue for the company apart from its e-commerce and cloud businesses. Revenue in Amazon's "other" category — which the company says is "primarily" made up of ad sales — more than doubled from 2017 to 2018, coming in at $10.1 billion last year, according to financial filings. The company's total net sales in 2018 amounted to $233 billion.

The study by eMarketer expects Amazon to grow its U.S. ad business by 50 percent this year. The research firm revised its previous estimates of Amazon's annual advertising revenue to account for new information about the company's "Other revenues" category, according to a spokesperson for eMarketer.

Revenue estimates for 2018 through 2021 were each raised by several billion dollars. The firm had previously expected Amazon's 2019 ad revenue to total $7.23 billion, but now expects revenue north of $11 billion, for example.

CNBC reported in October that some advertisers, particularly in the consumer packaged goods space, were moving as much as half their search budget from Google to Amazon. Advertising on Amazon can offer a strong return on investment, as customers are only one click away from purchasing the advertised product.

Of the top five companies in digital advertising — which also include Microsoft and Verizon — only Amazon and Facebook are expected to grow market share in 2019.

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