Amazon is taking a larger chunk of the U.S. digital ad business than expected this year, according to market research firm eMarketer.

The firm revised its annual U.S. digital advertising estimates, and now projects Amazon will rake in $4.61 billion in 2018. Previously, it thought the company was only going to make $2.89 billion.

More importantly, Amazon will take the third spot behind Google and Facebook. While Amazon will still only make up 4.15 percent of total U.S. digital ad revenue, it's growing while the other two digital advertising giants are decreasing. Emarketer expects the two will make up 57.7 percent of U.S. digital ad revenue (37.1 percent and 20.6 percent, respectively), down from 59.1 percent in 2017.

The increase is partially due to more people starting their online product searches on Amazon, rather than Google. Amazon has also grown its mobile advertising business 242 percent since last year, the report said.