Alphabet Inc.’s (GOOGL) Google and Facebook Inc.’s (FB) dominance of the U.S. digital advertising market is slowly beginning to unravel.

eMarketer forecasts that the combined U.S. digital ad market share of the two tech giants will fall for the first time in 2018, shrinking 1.7 percentage points to 56.8%. The research firm blamed this anticipated decline in a year when digital ad spending in the country is expected to grow nearly 19% to $107 billion on rising competition from smaller rivals such as Amazon.com Inc. (AMZN) and Snap Inc. (SNAP).

Google and Facebook, known as the digital duopoly for their stronghold over advertising customers, may view this loss of market share as a blow, even though they continue to comfortably rake in over half of the marketing dollars spent in the country. Combined, the two companies continue to increase their total ad revenues significantly, despite facing allegations that their websites unfairly interfered with political elections and are littered with fake news. (See also: Facebook Shares Fall After Data Leak Bombshell.)

According to eMarketer’s forecasts, Google’s U.S. revenues from digital advertising is predicted to jump about 15% to $39.92 billion in 2018, while Facebook’s ad turnover, led by a stellar performance from Instagram, is expected to climb 17% to $21 billion.

Those projections give Google and Facebook command of 37.2% and 19.6% of the market, respectively, down from 38.6% and 19.9% the prior year. This represents the first time that eMarketer has forecasted a decline for Facebook. Google’s market share contracted for the first time in 2016. (See also: Facebook, Google Dominate as Mobile Ad Spending Hits Record High.)

Amazon was identified as one of the companies responsible for chipping away at the digital duopoly’s market share. The retail giant is projected to bring in $2.89 billion in U.S. digital advertising this year, a 64% increase over 2017.

eMarketer noted that Amazon will capture 2.7% of the U.S. market in 2018, making it the fifth-largest digital ad player. By 2020, the research firm expects the retail giant to jump to third place, overtaking Verizon Communication Inc.’s (VZ) Oath and Microsoft Corp. (MSFT), with $6.4 billion in digital ad sales in the U.S.

The other name identified by eMarketer as a long-term threat to Google and Facebook’s throne is Snap. The social media company is forecast to grow its U.S. digital ad revenue by 82% to more than $1 billion in 2018, increasing its share to 1%.

The outlook for rival social media company Twitter Inc. (TWTR) is less promising, at least for the year ahead. eMarketer projects that Twitter’s U.S. ad earnings will decline for the second consecutive year in 2018, falling by 4.9% to $1.12 billion. However, the research firm described this downward trend as just a blip and expects the social media giant to return to positive growth in 2019.

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