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. 
Thats the plan of bitcoin doubler sites. They will pay you as long as you keep on deposit low amount. You will gain trust as you feel they really pay and double your coins. Once you deposit a high amount, they will never pay you or reply you back
Yeah that is a real fact of doubler sites as they will pay you once or twice but once they collect enough funds from all investors they will surely run away with it.