I wonder if people will just engage far less (or not at all anymore), which is less obvious on these graphs.

Say, if I check my FB twice a month to stay informed about events I might participate in, I'm still counted as a monthly active user, but I'm not active in a meaningful sense.

Similarly, many people might stop interacting with FB (which, if nothing else, takes time to register: at least a month).

Personally, I logged in for the last time in (I believe) 2012, but I won't delete my account just so my name stays "reserved".