I'm pretty sure the exchange price of Bitcoins is driven by the marginal supply and demand. So I think it's got to be what you call "investors" almost entirely

I'm pretty sure the exchange price of Bitcoins is driven by the marginal supply and demand. So I think it's got to be what you call "investors" almost entirely (note that not all people buying in have to be speculators: if lots of people suddenly decided they'd rather have some Bitcoins because they want to use them, that would suffice). So the question is how are the marginal supply and demand determined? The marginal demand is subject to spikes in interest, media coverage, the Cyprus crisis (I really doubt the latter is meaningful but perhaps it makes people everywhere think they want to move out of bank deposits. People certainly attribute things to it). The marginal supply is based on (1) mining, which happens at a fixed rate and (2) people who wish to exit Bitcoin, which is subject to lots of whims. Lots of evidence shows that most Bitcoins are hoarded, so it's possible that what we see is just a supply crunch - nobody wants to sell their Bitcoins and some people want to buy. I suppose the natural way to answer that is to go look at historical exchange volume across all the exchanges (or, as a first-cut proxy, just Mt. Gox).