This kind of hoarding is made more likely by the way Bitcoin is set up. Whereas the supply of modern, “fiat” currencies is controlled by central banks, the supply of bitcoins is permanently limited; there will never be more than 21 million bitcoins in existence. (The total number of coins is a result of the system’s initial rules governing how many bitcoins miners could earn, and how often.) Bitcoin’s limited money supply is one of the things that people like about it: the currency cannot be debased, as money can when central bankers print more of it. But the flip side is that if the demand for bitcoins rises, for whatever reason, then the value of bitcoins will necessarily rise as well. So if you think that bitcoins are going to become more and more popular, then-again-it’s foolish to spend your bitcoins today. The rational thing to do is hoard them and eventually sell them to new users. But that means there will be fewer bitcoins in circulation (and more in people’s virtual wallets), making them less useful as an actual medium of exchange and making it less likely that businesses and consumers will ever see Bitcoin as legitimate.

From Cryptocurrency - Technology Review

When bitcoin collapses, it will prove Milton Friedman right once more.