Wouldn't it be more accurate to call it a commodity? Stocks are (usually) stakes in a company, whose employees are actively trying to make a profit for the shareholders, whereas currencies just kind of... sit there (and move around, more or less randomly, at least, non-predictably).
i believe currencies are traded in the form of FOREX futures, which are contracts that must be settled on a date sometime from now. they are not mediated by an exchange like NASDAQ if memory serves, its just one big crazy open market.
i would say bitcoin trading belongs in the category of DIGEX then, a digital currency exchange.
And the best-performing asset for the last 2-3 years or so? Also pretty natural for high-tech IT company to allow this kind of thing.
Past performance is not an indication of future performance.