Until their HOA or their sports club start using Facebook as their official platform for announcements and communication.
I know there are other free platform that could fill the same role, but you will never find one where literally more than half the adult population already has an account and is familiar with the interface/features. Lots of groups use it because it's the path of least resistance.
Email is still the lowest common denominator.
Somewhere near 100% of internet users use email.
As ok123456 points out, there's a reason that you get asked for your email address and not your Facebook ID. Asking for Facebook leaves out a huge chunk of people.
I can barely get my D&D group to reliably use emails to communicate, instead of group texts that usually include the entire group, but not always.
It is top 3 most visited after google and youtube. Literally every other service/website you can imagine is worse in terms or popularity.
If >2.5B active monthly users is too low, I can't imagine that anything will ever be enough for you.
What? The over 40 crowd is FB's main demographic.