"12- to 17-year-old demographic declined by 9.9 percent in 2017" I'll offer those numbers are related to their parents already being there. Those kids will probably sign up or come back when they get a bit older.
When I joined Facebook (~2006/7) there was a very clear value to me: my and my college classmates could send messages, collaborate, and organize events.
Now it seems like a wasteland, I really have no idea how I could put Facebook to work for me, and since not everybody I need to collaborate with it is on Facebook (some are very opposed to it) it's useless as a platform for organizing events compared to other event-planning platforms. Why would anybody ever need Facebook?