I've noticed this amongst my family. The cousins, etc I have under 16 either don't have a facebook account or abandoned shortly after creating it. It's "uncool" because their parents see everything they do on there -- I know you can customize your sharing, but I doubt they know or care to be that careful on Facebook.
"This is why the smartest thing Facebook ever did was buy Instagram. Instagram is likely past a billion active users on its own now."
Unfortunately, this is why I don't use IG. I want to, but it seems impossible for me to keep it out of my Facebook like I want to. FB always pops up with "look what your friends are doing on IG" and I don't want my photography to be plastered in front of my grandparents.