I hadn't thought about it, but let's make a few assumptions and see how realistic that is.
* 7.7B people in the world.
* Subtract 1.4B people in China (even though plenty of those people actually have FB and login when abroad or via VPN, there are more countries where Facebook is blocked, plus outliers where usage is low).
* We don't want to count people who are really young. There's also a decay in popularity related entirely to age. I'm going to use ages 15-64, simply because the World Bank made it easy to get an estimate: ~65% of the global population[0].
* We have (7.7-1.4) x .65 = ~4.01B people between 15 and 64 years old in countries with Facebook. Given that Facebook growth skews towards countries with younger populations, I'd say this is a hard floor on what I'll call the "eligible Facebook population".
50% is high at first glance, but keep in mind that we've already excluded most of the low hanging fruit that would make it definitionally impossible for someone to use Facebook. I'd say their number is high, but not spit-take high.