Bitcoins are neither. They neither depend on more people joining,nor is the inherent value of a BTC dependent on newer adopters. These allegations of it being either a pyramid or ponzi scheme stem from the suddenly rising value of it,but that value inflation is due to pure demand-supply equations rather than anything else.
yep and since all world wide interest rates suck at this moment people are more willing to take a shot at something like this.
sell me a 30 year fed bond for 10% a year and I have no interest in BTC.
we have left a 30 plus year world of high interest rates.. getting 5-10% has stopped worldwide and people want something to put money into. a few years ago gold and silver were hot now this is hot. As long as easy interest does not exist there will be a substitute for investers .