I'm sure a good chunk would be fine with it. Facebook only maintains their social media dominance because the product is sticky: when your friends, family, municipality, favorite businesses, and clubs use Facebook and only Facebook for announcements, you kind of have no choice but to participate unless you're willing to work around inconveniences. Most users barely log in, maybe once or twice a month to check their direct messages and waste a few minutes on the feed.

It's a bit of a prisoner's dilemma: if other people use Facebook, they get to see things you don't and have an advantage in some domains. So a lot of people "use" facebook. But if all of the businesses and groups get forced off of Facebook they'll just find another way to make announcments.

Same thing with car ownership: if everyone stopped owning cars, everyone could happily get around with bikes, feet, and public transit. The world would quickly restructure to accommodate it. Sure, some things would be lost (it would be harder to go to remote places to hike, for instance), but other things, like the ease of getting around your immediate neighborhood, or easy access to stores, or polluting the air with less CO2, would balance that out in many ways.