The ad industry duopoly of Google and Facebook may not be all-powerful, but it is still feared by many advertisers.

In an April survey of over 500 advertising professionals worldwide conducted by Marin Software, 42% of respondents reported the dominance of Facebook and Google as one of the industry trends that will impact their business the most this year. Anxiety over the so-called duopoly outstripped concerns about other ad industry trends like the growth of video advertising, ad blocking and the rise of Amazon.

Given Google’s command over search budgets and Facebook’s over social media budgets, it is sensible for advertisers to fret that their industry is being swallowed by two companies. However, their share of the US digital ad market is actually declining slightly as their competitors' grow.

eMarketer estimates that Google and Facebook will capture a combined 56.8% of US digital ad investment in 2018, down from 58.5% last year.

Meanwhile, we forecast that Amazon’s ad revenues will climb 63.5% this year and that Snapchat’s will jump 81.7%. Amazon and Snapchat aren’t yet close to rivaling Google’s or Facebook’s ad businesses, as their combined share of US digital ad spending is still less than 4% right now. But their growth is worth monitoring.