There's no denying Amazon.com (NASDAQ:AMZN) has been a proverbial 800-pound gorilla in the room for some time now. From the biggest name in e-commerce to the leader of the public cloud market to the pioneer of ultra-rapid delivery, it's clearly been a disruptor.

Still, concerns that the company is running out of room to grow have been circulating for a while now. There's only so much stuff the world wants to buy online, and cloud computing competitors are chipping away at Amazon Web Services' market share. It's something shareholders should bear in mind.

The good news for its investors is, Amazon's got a real growth opportunity for its next era: advertising.

Amazon's growing ad business

Advertising isn't exactly a new venture for Amazon, but it has only come into view as a significant profit center this year. Last quarter, for perspective, the company's "other" line on its income statement -- which encompasses its advertising machine -- drove $3.6 billion in revenue, up 45% from last year's tally.

Picture of woman holding a smartphone, looking at an advertisment for a sale.

Image Source: Getty Images.

It wasn't a fluke either. Its "other" growth pace has been in the strong double digits for the whole year thus far, and eMarketer believes that sort of pace will persist at least through 2021. All told, in 2021, eMarketer believes Amazon.com will produce $16.7 billion worth of advertising revenue, consisting of $11.7 billion worth of search ad revenue and $5.0 billion in display ad revenue one sees at the top and sides of Amazon.com pages, at its wholly owned video gaming venue Twitch.com, or even on IMDB.com. That's well up from the $9.8 billion in total ad revenue eMarketer believes Amazon is on pace to generate this year.

That's not even the most interesting aspect of Amazon's ad business growth, however. Most interesting -- and encouraging -- are the digital ad titans Amazon is slowly displacing.

Amazon is an unexpected, potent rival

Don't worry that Facebook (NASDAQ:FB) and Alphabet (NASDAQ:GOOG) (NASDAQ:GOOGL) breadwinner Google are facing an existential crisis. They're not. Both are still the biggest names in the business globally, claiming the second and first spots, respectively. Even Alibaba is in the mix at a distant third behind Facebook. Amazon is a similarly distant fourth.

None of those bigger online advertising middlemen can afford to ignore Amazon, however, for one overarching reason. That is that Amazon is growing its digital ad business at a faster pace than any other company, with its advertising revenue projected by eMarketer to expand by 33.1% this year. The next-nearest player is Facebook, but even then, it's only expected to see growth of 26%. Twitter ranks third with its 18.7% ad revenue growth projection, followed by Google's 17.6%. Comparable year-over-year growth rates are expected to hold through 2021.

Amazon is garnering business other companies in the arena may have expected to split up between themselves in the absence of a newcomer.

Worth the wait

It's still not a game-changer for Amazon, which is expected to produce upwards of $275 billion worth of revenue this year. This year's $9.8 billion in ad revenue won't move the needle much. Even at the expected 2021 tally of $16.7 billion, its advertising arm won't make or break the organization.

For investors who can wait another couple decades, though -- as many did when Amazon was a fledgling e-commerce name in the late 1990s -- a booming ad business isn't out of the question.

Amazon is certainly positioning itself as a platform that can connect consumers with advertisers. While plans to enter the low-margin grocery store arena aren't exactly cheered by all, there's more to the matter than merely selling groceries. Grocery shoppers contribute to their digital profiles when they make purchases, and sooner or later, Amazon will find a way to feed them digital ads. Its Alexa-powered Echo devices and Prime Video content are also ultimately means of building digital profiles of individuals, even if it takes years to gather enough data to meaningfully market products and services to those people.

Sure, Amazon's e-commerce and AWS growth might be slowing. The company's clearly got new growth opportunities ahead though. At stake is a piece of a global digital ad market that will be worth more than $300 billion this year. Even a relatively modest piece of that pie could prove to be a big deal for the company.