A ponzi scheme means it can only work if new people keep investing fresh money to sustain the pyramid. Bitcoin will be useful even at $1, so it is not a ponzi or a pyramid.
Also a ponzi scheme usually guarantees profit (like for example 5% monthly), bitcoin doesn't guarantee any profit.
Bitcoin needs new investors in order to sustain the price, otherwise it will go to 0 easily... just remember that there is 13 million Bitcoin in existence against a really thin order book, a 100K market order across all exchanges could easily bring the price to zero....but thankfully more than 90% of holders thinking about getting rich and hoarding their coins causing a balance between the liquidity and the lack of demand, which bring some kind of stability (or rather a slow price decline).
I still think that the price was a huge bubble that will burst, but I just wish it will continue doing this calmly without any sort of panic and massive flash crashes that could lead to even more panic and shake the confidence ... well talking about confidence, the get rich quick and pigs/perma-bulls/perma-bears are going to be burned and will lose allot eventually, but the objective investors/watchers/entrepreneurs who really understand the technology and what it really offers and what need to be improved will benefit the most.
