The social network made major changes to its News Feed in January, including cutting down on branded viral videos, but the drop in usage came before these changes were implemented.
Time spent on the social network fell by about 5%, or roughly 50 million hours a day, in the last three months of 2017.
Facebook founder Mark Zuckerberg said it had been a “hard” year for the social media giant. Shares in the company fell 5% in after-hours trading.
Full-year profits rose 56% to $15.9bn and revenues climbed 47% to $40.7bn while fourth quarter revenues and underlying earnings beat analysts’ expectations.
But a 14% rise in the number of daily active users to 1.4 billion in December compared to a year before fell slightly short of Wall Street expectations.
Mr Zuckerberg said: “2017 was a strong year for Facebook, but it was also a hard one.”
Facebook had warned earlier this month that tweaks to its news feed feature would result in a hit to user engagement and it has also been affected by changes to the way it shows videos.
Zuckerberg said: “Last quarter, we made changes to show fewer viral videos to make sure people’s time is well spent. In total, we made changes that reduced time spent on Facebook by roughly 50m hours every day.”
He argued that it is “encouraging meaningful connections between people rather than passive consumption of content” and that by doing so it will be stronger in the long term.
The changes will also see it highlight “trustworthy” news in its news feed following claims that Russians and others spread false reports on the site, particularly during the 2016 presidential election.
Mark Zuckerberg: Facebook changes reduced time spent on site by 50 million hours a day in Q4 from CNBC.