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…
Google Trends never lies. Some say that people have learned they don't have to search for Facebook, but the trend for Facebook follows the 'myspace curve of disengagement':
https://trends.google.com/trends/explore?date=all&geo=GB&q=f...
Note that people never type 'whatsapp' into a browser.
I think that social media is always going to be fickle. Google and search is a much better bet for the product being relevant in years to come.
Facebook is also a black hole. It is very rare that something written on Facebook is noteworthy enough to be shared outside of Facebook, here for instance.