Facebook's move last year to discourage passive consumption of content, especially videos, has impacted engagement, said the report by research firm eMarketer.
This year, US adult Facebook users will spend an average of 38 minutes per day on the platform (on all devices), said the report.
"Facebook's continued loss of younger adult users, along with its focus on downranking clickbait posts and videos in favour of those that create 'time well spent,' resulted in less daily time spent on the platform in 2018 than we had previously expected," eMarketer principal analyst Debra Aho Williamson said in a statement.
"Less time spent on Facebook translates into fewer chances for marketers to reach the network's users," Williamson said.
Engagement on Snapchat, meanwhile, has essentially plateaued. Instead of growing, time spent among users fell slightly last year because of lingering fallout from the app's failed redesign and competition from Instagram.
"We now expect time spent among Snapchat's adult users to remain at 26 minutes per day through 2021. Our previous forecast projected 28 minutes per day in 2019," eMarketer said.
The picture is somewhat brighter at Instagram, where user time spent is still growing. Average daily time on the Facebook-owned platform will reach 27 minutes this year among US adult users, according to the forecast that added the time spent would increase by one minute every year through 2021.
"Features like Stories, influencer content and video are all contributing to more engagement and a slow but steady uptick in time spent on Instagram," Williamson said.
( With inputs from IANS )