So, not to discredit this number, but here are some things to consider -

1 - Bots. Lots of bots.

2 - Multiple accounts. I've seen people with as many as 4 accounts, used for different purposes(friend groups).

3 - 'Facebook is the internet'. Facebook is free(ie not charged as data usage), and therefore, 'the internet', in many poor nations. I'm not actually sure if this is Facebook's doing, a carrier agreement, etc, but it exists. This accounts for what I assume are the bulk of the 'real' users.