I think this is going to make facebook billions, and has a particular advantage vs iCrap because it is device agnostic.
The 'make or break' of the platform will be how easy it is to download your tracks to a portable device. If Facebook get this right, they've essentially delivered the ultimate cloud-based music platform, facilitated the legal sharing of music, removed the requirement to use a proprietary device, added the social element - and have the secret X factor, which is the viral element.
Companies like Zynga have made their fortunes because games on Facebook are viral and post constant updates to one another. It is reasonable to deduct that people will be more encouraged to buy music that their friends are listening to and Facebook is the number 1 platform to view that...