The CEO of Spotify recently said that 'On certain days, we're consuming more Internet capacity than Sweden has as a country.
Spotify uses a P2P setup that takes bandwidth usage away from a single server setup allowing the network to grow larger. P2P works by users instlaling the Spotify application which then streams information from one computer to another rather than from a central server setup.
It's interesting to see how quickly Spotify have grown as they now have 320,000 paying subscribers with around 7 million users in total and yet, they have not entered the US market.
Reasons for not entering the US market are down to it being so huge with a lot of deals to be struck up with music providers. Once all this is ironed out it is expected to launch for around $10/month out that way.
When opened up in the US it makes you wonder how much bandwidth will be used in total as the US is potentially a huge market for this sort of application. iTunes may have a serious competitor!