First voter gets their money back with the 5th voter. But the 5th voter only gets their money back with the 69th voter.
The first voter doubles their money with the 32,769th voter and the second voter doubles their money with the 33,554,434th voter.
So doubling the money is very unlikely for even the first voter and impossible for everybody else.
So if I'm honest, I don't think this algorithm will provide much incentive to vote.
the upvotes are not considered an investment but a social game
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It seems like 20-30 bitcoin upvotes happen a day.