Tulip mania wasn't a Ponzi Scheme... it was just a bad investment.  Ponzi means old investors are paid off with money from new investors, to give the appearance of a monthly return, but in reality, there is no investment at all, just shuffling money around.  So, like Tulip Mania and The DotCom bubble, bitcoin can't be a Ponzi because there is no one offering huge returns or monthly payments, who is going to disappear with most of the "investment" money.