U.S. digital ad spending surpassed $100 billion last year, according to the Interactive Advertising Bureau's latest full-year report. The $107.5 billion marketers spent on digital ads last year was up 21.8% from 2017.

The top 10 digital advertising companies accounted for 75% of total ad spend, up from 72% in 2017. Despite a focus on the duopoly -- Facebook (NASDAQ:FB) and Alphabet (NASDAQ:GOOG) (NASDAQ:GOOGL) subsidiary Google -- losing market share, ad spend is increasingly concentrated among a few winners.

Investors interested in capitalizing on the continued growth of digital advertising need to consider the trends driving the overall growth, as well as the companies best-positioned to win. And based on IAB's findings, it's likely the companies already winning.

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Image source: Google.

Two big trends in digital advertising

There are two things that stand out in IAB's report.

Video advertising is increasingly popular, with spend up 37.2% from the previous year to $16.2 billion. Video ad spend accounted for 15% of all digital advertising in 2018, up from 13% in 2017.

Additionally, ad budgets are still shifting to mobile. Mobile accounted for 65.1% of all digital ad spend last year compared to 56.7% in 2017. Overall mobile ad spend increased 39.7%.

Mobile video ad spending grew 65.2% year over year.

Google's YouTube and Facebook are dominant forces in mobile digital video advertising. Nearly all of Facebook's 2.7 billion active users across its family of apps access it through mobile. YouTube says it has 1.8 billion monthly active users spending an average of an hour per day streaming video on mobile.

Facebook generated $6.8 billion in total digital video ad revenue last year, according to eMarketer. YouTube kept about $3.4 billion of the video ad spend on its platform, which it splits with creators.

Both companies are poised to keep winning the vast majority of video ad spend as they invest in video content. YouTube recently announced plans to offer its YouTube Premium originals for free with ads, and Facebook is investing hundreds of millions in original video as well. Facebook is also pushing video ads in its Stories products.

Other companies doing well with mobile video include Twitter (NYSE:TWTR) and Snap (NYSE:SNAP). Twitter has made a lot of investments in content over the last few years, and video advertising now accounts for the majority of its ad revenue and was the company's fastest-growing ad format last year. Meanwhile, Snap has recently seen success with its nonskippable six-second video ad format in Snapchat's Discover section. Twitter and Snap generated an estimated $633 million and $397 million in U.S. video ad revenue last year, respectively, according to eMarketer.

Let's not forget about Amazon

Amazon's (NASDAQ:AMZN) burgeoning advertising business has made a lot of headway over the last couple years. Its growth has largely been fueled by sponsored product listings in its search results, but the company is also turning to digital video advertising as a source for additional inventory.

The company launched IMDb Freedive earlier this year and it reportedly has plans to expand upon the ad-supported video-streaming service. Additionally, the e-commerce giant is experimenting with video ads in the search results of its mobile app.

Amazon is already the third-largest digital advertising company in the U.S. behind Facebook and Google. While it's not going to surpass the two giants anytime soon, it should be able to continue taking market share as it leverages its retail platform and smart device business to create new ad products and find new inventory for marketers to fill.

Why things will get more concentrated

Ad spend is likely to become increasingly concentrated among a select few companies going forward as marketers look for mobile video ad inventory. Developing a large mobile audience is extremely difficult; consumers don't want to install yet another app.

What's more, video content to advertise against can be very expensive. Amazon spends billions on video content every year. Facebook is spending hundreds of millions on video content to seed its Watch platform. Twitter notes video content costs are a main element in the growth of its cost of goods sold.

These factors ensure the rich will get richer. So investors interested in digital advertising would do well with one of the market leaders.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.