I don’t know if they still do, but Facebook used to count viewing a web page with a Facebook “like” button as being an active user

Ok my bad. But what do they count as activity would also be a good question. Posting? Just opening the app? What constitutes activity to Satan Zuckerberg? I wouldn’t be surprised if just accidentally opening the app even if you’re never posting or reading counts.

Monthly active (non-human?) users?

I think that was understood, and I’m also of the opinion that Facebook is full of shit.