For subscription to work, you either:

1- undercharge users from wealthier countries

2- price poorer users out of you platform

3- give up on the idea of worldwide adoption (Facebook scale, as you say) entirely

4- attempt to charge different amounts by country of origin, and watch your users cheat the system mercilessly

5- go freemium, and suffer the same fate that news organizations do- find that far too few are willing to pay to go ad-free, stick ads back into the free version, and end up leaking data anyway

I suppose 4 might be the most feasible option, but once it is obvious that some people pay more for the exact same value, they are likely to assume that the product has less value than it actually does, feeling that they are being ripped off.

In short, there is probably a good reason that paid services will never reach Facebook scale.