The number of people in the U.S. and Canada who check Facebook every day dropped between the third and fourth quarter of 2017, the first such quarterly drop in company history.

Facebook usage in North America has been largely flat for the last several years, leaving international growth to pick up the slack. But the drop suggests that Facebook usage has reached a saturation point in its first and most lucrative market, and could foretell similar usage drops around the world.

The drop could also be a result of a steady drumbeat of negative press about Facebook, including concern over foreign governments using Facebook posts to divide the U.S. electorate during the 2016 election season, and growing concern over violent and other inappropriate content on the site.

If usage flattens, Facebook will have to pack more ads into the News Feed, charge advertisers more per impression, or figure out new and similarly profitable business areas to maintain long-term growth.

Facebook claimed 184 million daily average users (DAUs) in the U.S. and Canada in Q4 2017, down from 185 million in Q3. Those users accounted for $6.39 billion of Facebook's revenue in the fourth quarter, up from $5.03 billion in Q3. That revenue increase despite the usage drop is probably thanks to seasonal changes related to the ad market, which is typically strongest in the holiday quarter.

On the company's earnings call, CFO David Wehner said that usage in this region would "fluctuate" based on m arket penetration.

Here's the usage chart from Facebook's earnings deck: