That’s according to new estimates published Wednesday by research firm eMarketer. The firm said Amazon’s “meteoric rise” would continue “to fuel speculation about whether the digital retail powerhouse is a genuine rival to the biggest ad publishers in the world.”

U.S. advertisers are likely to spend about $4.61 billion on Amazon’s advertising platform in 2018, which eMarketer says is a whopping 144.5% increase from the previous year. The research firm said that while some of Amazon’s ad growth is due to an accounting change, the primary takeaway is that companies are increasingly buying ads on Amazon.

Because of its growth, Amazon has surpassed both Microsoft and Verizon’s Oath conglomerate as the third-biggest digital advertising firm in the U.S.

One of the drivers for Amazon’s growth, eMarketer said, is that advertisers know that people who visit the online retailer do so to shop; they aren’t merely browsing the web or reading posts from families. It makes sense that an advertiser would want to run ads hawking its product on a service dedicated to the sole purpose of shopping.

Still, Amazon’s online ad business is a baby compared to Facebook and Google, which together account for 58% of the overall U.S. online advertising market. The research firm previously said that Facebook’s U.S. online ad business would make $21 billion in 2018, while Google would make $39.92 billion.

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Amazon is not likely to significantly encroach on Facebook and Google’s so-called duopoly, in “the short term,” eMarketer said.

“But Amazon has the ingredients for a truly powerhouse digital ad business, the key to which is doing more than just selling ad impressions,” the firm said.