Note: personal opinion bellow It seems to me that the overall interest in Facebook is decreasing. The social network hasn't had any interesting feature added to it in the last couple of years. It's becoming boring and boring, so that's why I believe people are leaving. Still, Instagram and WhatsApp are running strong with barely no competition. We don't see any news about their user base decreasing and news channels…
If I recall, correctly, the last major feature (read: that had any fanfare) was when they added the ability to have hi-res photos (and more of them), which was timed with the release of the Transformers movie? So, yeah, it's been a hot minute since they did anything substantial.