You can't eat a smartphone either but all of these theoretical people with zero disposable income seem to have them for some reason.

and if you can't afford $1 / year FB doesn't give two shits about you because you have no disposable income anyway.

They probably do care, indirectly. They want as many people as possible on their service. Maybe you don't have $1/year for Whatsapp and therefore aren't of much interest to Facebook for advertising either, but your friends' friends might.