For the first time in years, tech behemoths Facebook Inc. (FB) and Alphabet Inc.'s (GOOGL) Google, having earned the title of "the digital duopoly," could see their share of the U.S. digital advertising market decline, as reported by the Wall Street Journal. 

Mark Zuckerberg's Facebook has seen its stock skyrocket 558% over the past five years, while search giant Google shares have returned 96% to shareholders over the same period and the S&P 500 Index has increased 75.4%. While Facebook stock has only been trading on the public markets since 2012, Google stock has surged over 900% since 2004 and its initial public offering (IPO) price of $85. 

Smaller Players Set to Grab Market Share

A recent report by market research company eMarketer suggests that as the old market leaders are challenged by a growing number of rivals, such as e-commerce giant Amazon.com Inc. (AMZN) and social media platform Snap Inc. (SNAP), the combined companies will continue to lose ad dollars to competitors even as the total opportunity increases. While the two are still the clear market leaders, with no other competitor holding more than 5% of the market, their crumbling duopoly could present risks to the tech giants' earnings, revenue and stock growth in the long-term. (See also: Facebook and Google’s Days Are Numbered: Soros.)

"The two companies will capture a combined 56.8% of US digital ad investment in 2018, down from 58.5% last year," according to the eMarketer report. "Smaller players such as Amazon and Snapchat are experiencing faster-than-expected growth." The research firm's previous forecast in September predicted that both Google and Facebook would post a slight increase in total market share for 2018. 

As total digital ad spending in the U.S. is expected to jump nearly 19% to $107 billion this year, Google's ad business is expected to grow 15% to $39.92 billion, while Facebook's is forecast to increase 17% to $21 billion. At the same time, Amazon is expected to grow its digital ad dollars 64% to $2.89 billion, reflecting 2.7% of the total market. Within just two years, eMarketer projects the retailer will beat out Verizon Communications Inc.'s (VZ) Oath and Microsoft Corp. (MSFT), to reach third place with a $6.4 billion ad business. 

As Facebook's News Feed ad prices reach their limit, its Instagram platform should be its saving grace and "quickly become the engine that drives growth for the whole," wrote eMarketer's senior forecasting director Monica Peart. Snap, which has seen its stock struggle since its IPO a year ago on fears of Instagram's booming popularity, is expected to grow its U.S. digital ad revenue by 82% to over $1 billion in 2018, bringing its total share to 1%.

Things aren't looking so good for Jack Dorsey's Twitter Inc. (TWTR), either, which is forecast to see a 4.9% decline in U.S. digital ad revenue to $1.12 billion this year before returning back to growth in 2019. 

The eMarketer report comes amid heightened tension between brands and the "digital duopoly," which has been criticized for failing to address concerns with clients, including getting rid of inappropriate ads and fixing measurement discrepancies. Google and Facebook have also seen their images tarnished by the media, activists, their own users, and others calling for more stringent regulation surrounding various issues such as the negative effects of social media on youth and Russia's role in the spread of misinformation to influence the 2016 U.S. presidential race.  

Zuckerberg's Facebook has seen its shares plunge this week, with Monday serving as its biggest one-day decline in four years, as news broke regarding a scandal involving data analysis firm Cambridge Analytica. The company reportedly worked with the Trump campaign using information improperly obtained on 50 million Facebook users. Many on the Street expect a potentially severe backlash by the government and the platform's users to cast a shadow over the firm's long-term growth story. (See also: How the Facebook-Google Duopoly Crushes Rivals.)