People don't use Facebook, hence you find them on other platforms. They may be thinking that if they don't use it, then nobody does.

But you also have to consider where these people live. It might be "dying" there, while Facebook is getting new users in other markets. I remember watching a video about genocide in Myanmar and the role that Facebook played. Access to Facebook was free if I recall correctly, it was the internet for most people.

It'd be interesting to see data by country.