Here’s a troubling data point if you’re a Facebook investor: The company may have finally tapped out its most valuable market, the U.S. and Canada.

Facebook’s daily active user base in the U.S. and Canada fell for the first time ever in the fourth quarter, dropping to 184 million from 185 million in the previous quarter.

People are spending less time on Facebook — and CEO Mark Zuckerberg claims that’s by design.

Zuckerberg said that people spent “roughly 50 million hours” per day less on Facebook last quarter. Zuckerberg said that “we made changes” in order for that time decrease to happen, and added that “focusing on meaningful connections, our community and business will be stronger over the long term.”