I’d be beyond shocked if even one percent of all Facebook users would pay $5/month for it. I think the only population of users who’d consider doing so are the intersection of those who are affluent (by Western standards), are extremely opposed to Facebook ads, and use it enough to consider paying for it.

I’d be surprised if you could get 1% of the US population of users to consider it, let alone the global population. You’d realistically be looking at single to very low double digit millions of users at the maximum.