Amazon.com (NASDAQ:AMZN) is getting serious about its advertising business, and owners of Amazon stock should keep an eye on the ad unit.

AMZN got into the advertising business a few years ago, but when the e-commerce giant started to aggressively pursue the opportunity last year, it was a huge clue that the company was going to get serious about advertising. Amazon doesn’t do anything halfway.

If there was any lingering doubt, though, it was wiped away last week. That’s when advertising market research firm eMarketer proclaimed that Amazon had become the United States’ third-biggest digital advertising middleman.

Though Amazon is still well behind Facebook (NASDAQ:FB) and Alphabet (NASDAQ:GOOGL, NASDAQ:GOOG) in advertising, it has passed Verizon Communications (NYSE:VZ) property Oath — essentially a combination of Yahoo and AOL — in the still-growing online advertising market.

Amazon’s ad business thus far hasn’t been a major factor in Amazon stock price, nor is Amazon stock moving much in the wake of eMarketer’s report. Nevertheless, the emergence of AMZN’s ad business is a development that current and would-be owners of AMZN stock need to keep on their long-term radars.

What eMarketer Said

eMarketer wasn’t coy. It began its report by stating that: 

We estimate that U.S. advertisers will spend $4.61 billion using Amazon’s platform this year, a 144.5% increase from last year (with some of the growth due to an accounting change made by Amazon this year). That will give Amazon a 4.1% share of the US digital ad market-leapfrogging Microsoft and Oath to make the etailer the No. 3 digital ad firm in the country this year.

For perspective, Facebook and Alphabet’s Google jointly control 57.7% of the United States’ digital advertising market. Given Amazon’s growth in this area, though, it’s expected to account for 7.0% of the country’s digital ad market by 2020.

That growth will come at the expense of Facebook and Alphabet. By 2020, their combined share of the market is expected to be pared back to 55.9%.

Amazon hasn’t confirmed or denied the accuracy of eMarketer’s estimate of the revenue generated by Amazon’s ad business. AMZN CFO Brian Olsavsky only explained during July’s conference call that “It’s now a multi-billion dollar business for us,” adding that ads were starting to “make an impact” on the company’s bottom line.

The History of Amazon’s Ad Business

Amazon has slowly but intelligently developed an advertising business that looks a great deal like its rivals’ ad operations.

The stage was set as far back as 2014, when the e-commerce company first rolled out a self-service advertising platform. It was the foundation for an idea that would eventually become known as “programmatic” ads, which are also now offered by Facebook and Alphabet. Amazon upped the ante last year, opening up the platform further by allowing agencies to resell Amazon’s available ad inventory.

Each such evolution has made the company’s advertising platform more effective for marketers, posing an increasingly legitimate threat to the dominant players in the business. eMarketer’s outlook implies 50% growth in Amazon’s ad revenue every year through 2020. If eMarketer’s estimates are on target, Amazon’s ad revenue will reach more than $10 billion by 2020.

That said, Amazon’s move into deeper advertising waters may not prove quite as easy or as fruitful as some owners of Amazon stock are hoping.

As Recode noted earlier this month,  sponsored ads completely obscure some products from shoppers initially.  As a result, some shoppers only see products that are being promoted by sellers who have bought ads. While sellers who have bought ads should be prioritized, consumers may find  these paid promotions overwhelming.

However, this situation hasn’t been problematic yet.

Bottom Line for Amazon Stock

Even if eMarketer’s outlook is on target, this development won’t be a game-changer for Amazon stock… at least not in the near future. The firm’s 2020 ad revenue estimate of a little more than $10 billion is only a fraction of the $208 billion of revenue that Amazon has generated over the course of the past four quarters.

And it would be naive to think that Facebook, Google, and the Yahoo/AOL duo won’t somehow fight back.

Nevertheless, given Amazon’s sheer reach into so many different facets of consumers’ daily lives, it’s difficult to think the company won’t at least leverage its advertising platform into a machine that draws more consumers and companies into its ecosystem. Once that goal has been accomplished, those new customers can be monetized in other ways. Over the longer term, the direct and indirect benefits of the ad business could have a significant, positive impact on Amazon stock price.

As of this writing, James Brumley did not hold a position in any of the aforementioned securities. You can follow him on Twitter, at @jbrumley.