Here is what I honestly think happened: a lot of older gen x and boomers saw their reputations destroyed on Facebook during the Trump Era.
The people who didn't leave Facebook because of them just put them on mute. They only had other old people to communicate with. This didn't satisfy them though, because really their entire ideology is wrapped around triggering other people.
So they went to reddit and discovered that anonymous shit posting was safer and their Facebook went back to livelaughlove largely.