the real test of a bubble is to see if people are buying to hold or merely flip. their time horizon is also an important consideration.
if they're buying to flip with a short horizon, it's a bubble. the first buyer to feel they might be the greatest fool walks away, and the bubble pops.
> speculation driven
speculation is a normal economic mechanism. taking risk in expectation of reward.
> you can't have an exchange-based economic system without some expected stability in the value of the exchange
yes you can. especially if the value of the currency is only going up. probably not if it's going down.
currently the ratio of stock to flow is rather high. since there is less flow available, and lots of fiat chasing it, the price rises. since the value of the currency rises, people who sell in that currency benefit (especially if they keep the difference in bitcoin).
money is a shared societal delusion. it's a giffen good, where rising price acts as a signal that more should be owned. misesian/mengerian theory requires only an initial direct barter exchange, the rest of it is bootstrapped by historical events (given certain basic properties, which bitcoin has in abundance[2]).
we are living those historical events today, thanks to the ECB & Cyprus.
at the moment bitcoin is a transient medium of exchange (usd->btc->silkroad) on both sides and a few risk-takers are betting on it holding value, where few is relative to world population. most merchants (at least thru bitpay etc) don't hold it either.
for the price to drop consistently, bitcoin-busters would have to take a large position, driving up the price, and then consistently sell enough to trigger other people's latent limit-sell orders. the problem is that naked shorting is not possible, so on the way to building large positions, they will send out the rising-price signal, which will make downward price-pressure quite hard later (aka high prices will be sticky since people's expectations have been reset). [1]
when the price gets high enough, the realization will sink in that the only way to accumulate bitcoin is to participate in the economy with one's labor (ie, sell things for bitcoin).
that's the ultimate win for bitcoin.
[1] oddly, one of the criticisms of btc, namely that Satoshi holds a significant amount, will provide downward stickiness as long as that large cache isn't sold.
[2] bitcoin has a lot more properties than most other monies, principally anonymity & distance-agnosticism, and also invisibility via the use of darknets/tor