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An interesting method in trying to figure it out, I saw this question and wondered if anyone would be able to give a more informed answer so think this is a step in the right direction. Maybe the best we have at the moment is communities as you say.

I don't think anyone will be able to answer this truly though as there it's not logged anywhere I believe, maybe by the Dapps, but not in the blockchain itself.

You just can see the volume of posts. Currently Korea, Latin America and Indonesia are strong
Screenshot from 2018-09-01 00-52-32.png

But you can't know exactly.. maybe you can compare the "steemit" popularity on Google trends