3b monthly users would mean nearly half of the entire world's population is on Facebook at least once per month. No matter how you shake it, that's simply not a realistic number, even before you consider people that live in areas with no internet, or regimes that disallow sites like Facebook, children too young to even possibly use Facebook, so on.

It wouldn't surprise me if the total number of real users, not counting bots and alt accounts, is probably ~1b or less.

That's not growth, it's just logged in users navigating the web and FB counting anybody that hits their APIs. Facebook is dead.