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Very low. Way less than 1%

I think it's around 2%, remember this is for developed nations only, and adult population. See my other post: 0.92% of U.S. adults are on Binance.

What's the actual answer? Does anybody know for sure?

Where is less then 0.01%, most of the population is very poor and have no money to invest in crypto. Of those who aren't poor maybe 50% know of crypto, and of that 50% maybe 2% are currently invested. So the total population of the world invested is basically nobody.

If you include no developed countries maybe 0.00001% of the population is invested in crypto.