Email, SMS, good ol' phone calls, Signal, <insert local app/platform here>, your own website, etc, on top of whatever you use right now.
If you're in a country that relies a lot on Facebook or Whatsapp, that's where the main focus will be, but at least try to have alternatives just in case something goes wrong.
So 4/4 of those are platforms controlled by a single company or a few large corporations. This really isn't a win in any meaningful sense.
It should be fine for huge corporations to exist and provide services really efficiently at scale while also being forced to play nice and respond to the will of the people they serve.
If we collectively can't stop Facebook from doing bad thing and being bad stewards to their own platform then you won't be able to stop whatever would replace them either.
