It'd be easier for Facebook themselves to estimate, because they have the affected set per-country (userbase), and they know the baseline per country (published

It'd be easier for Facebook themselves to estimate, because they have the affected set per-country (userbase), and they know the baseline per country (published national statistics - both users and non-users). They've probably done that research, and they probably also know whether people engaging in affairs are more or less likely to increase engagement with the platform and click on related targeted ads and make relationship-relevant (or whatever the internal terminology is) purchases. Whatever the results, they can forge a kind of internally-consistent worldview. "This is what we've seen about our users, so this is what we think and will say humans are like naturally to avoid cognitive dissonance in the ranks and ourselves"