With all these doubler sites that claim to do it I don't think it keeps paying because of the last few that joined will lose out by not getting paid when they close up shop as they always seem to do when they want to run away will all the investors funds. 
That is a strategy to make them lose more. By just making you feel that you are doubling it, well they will give it at first then they will give more encouraging you to keep doing it and making the investment larger for a bigger profit until they will bail out when they see profit.