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