LOL, like two tween-age girls sharing a pair of headphones. Besides on the radio or at a concert, have you EVER even been cognizant of listening to a song at the same time as a friend, much less thought to yourself "It would be totally cool if Jim and I could listen to this awesome Jonas Brothers song at the same time while watering each other's crops!!!".
Integrating a pay service into Facebook might not go so well. Sure, you can buy facebook credits if you want, but for the most part, everything on facebook is "free". Not sure people would be willing to pay for Spotify just to listen to a song "simultaenously" with their friends. Not saying it's a bad idea if you plan to use Spotify anyway, I just don't see how it fits with Facebook if you have to buy a subscription.
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...