U.S. total digital ad spending is poised for a 16 percent growth this year to $83 billion with Google taking the largest piece of the pie, but Facebook is fast catching up, according to eMarketers' latest forecast.

According to the research firm, as the overall digital ad industry expands, Google will continue its dominance, but its share of the U.S. market share will shrink to 40.7 percent at the end of the year, as Facebook's reached nearly 20 percent. 

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Moreover, eMarketers predicts that Google's revenue from digital ads in the U.S. is expected to grow by only about 15 percent this year, while Facebook's will rise by a whopping 32 percent.

Social media giant Facebook is leading the display business with a 39 percent market share as revenues from this segment is estimated to jump by 32.1 percent to $16.33 billion. Google's display business, on the other hand, is expected to only reach $5.24 billion with its market share dropping to 12.5 percent.

Google, meanwhile, is expected to lead the search market, claiming around 78 percent of the total search ad revenue. Its market share is expected to expand by 16.1 percent to $28.55 billion this year.

According to eMarketer, Facebook attracts advertisers both in terms of usage and engagement, thanks to its active promotion of new types of video content which encourage users to spend more time on the social media. 

"Facebook's users are increasingly captivated by videos on the platform-not just on Facebook, but on Instagram as well. Video, both live and recorded, is a key driver of growing user engagement and advertiser enthusiasm,"  said eMarketer forecasting analyst Monica Peart.

Snapchat is also poised for tremendous growth. eMarketer predicts the company's ad revenue will skyrocket by 157.8 percent to $770 million in the U.S. 

Nevertheless, its ad share will only take a small piece of the pie at just 1.3 percent this year. It is estimated to grow to 2.7 percent by 2019.