The "many of whom have no idea they are on free basics" part is a little weird to me. Are these "users" like my father, who technically has internet access but doesn't use it?

Aside from that, I don't think Facebook needs to find "new" internet users to be providing access to the needy. If the users' previous access was the internet equivalent of a polluted river 15 miles away, building a clean well right in the village is still a benefit, even if they're only allowing livestock with an odd number of toes to be watered, which everyone admits is kind of weird.