Ponzi schemers or CON men or woman are tricksters who pretend they have investments so that people can give them their money. Usually schemers are very well liked and respected throughout their community because you can't scheme someone who doesn't trust you, if they don't trust you they won't give you their money. 1st they pretend like they have a great investment and tell people they'll make a large amount of money if they invest(there's really no investment). The more people that trust the schemer with their money, the more investors the schemer will get and with the money they get from their new investors they pay their old investors.
I think that Madoff was able to keep his scheme going so long because he had so much investors that were putting so much money that he was able to pay most of his investors. So because he was able to pay most of his investors with large sums of money they really didn't expect that he didn't have an investment because how else would he be able to pay them their money if he didn't.