Forrester Research released yet another set of predictions forecasting the near-term effects of several of advertisers’ largest concerns, noting that ad spend on Amazon and mobile devices are set to increase while fraud, ad blocking and brand safety concerns will continue to influence spending.

According to the “2017 To 2021: US Online Display Advertising’s Great Reckoning” report, Amazon’s ad revenue will pass $2.5 billion by 2021, though other guesses already place the company’s earnings at $3.5 billion for this year alone.

Forrester predicts that this growth will come, in part, at the expense of Facebook and Google, though the total value of Amazon’s ad business is so minuscule by comparison that it’s unlikely to serve as a credible threat to the “duopoly” just yet.

Even if Amazon joins Facebook and Google’s playing field, it’s unlikely that the e-commerce giant will affect their “walled-garden” behavior in any significant fashion, given that it already acts just as, if not more, secretive about its advertising business.

While Amazon is predicted to eat up some of Google and Facebook’s share of consumer product advertising, Forrester predicts the duopoly will see significant growth on mobile. The report anticipates that mobile spending will shift increasingly to in-app ads, reaching 66 percent share by 2021. This bodes well for the pair of advertising giants, as both control the majority of the most-used apps in the ether.

According to Forrester, the shift will be driven, in part, by concerns about ad fraud and blocking, which will consistently waste 33 percent of all digital display ad dollars through 2021. Programmatic exchanges will suffer at the hands of smaller groups of more reliable private and direct-trading platforms, the firm predicts.

Though ads placed on Facebook, Google and Amazon may be more expensive than other programmatic options, marketers will opt to pay more for reliable measurement and safety.

“Marketers want more visibility on the inventory they buy and are willing to pay more to appear in a brand-safe environment that promotes viewability, controls fraud and makes sophisticated use of data for targeting,” the report reads. “The state of ad fraud and viewability in open programmatic will push them to implement tighter guide rails around their media buys.”