Why would they incorporate bitcoin? I believe they'd rather develop some other proprietary payment system or cooperate with regular payment providers.
I see this being a far more likely outcome, unless they can set it up to be taking a transaction fee on each purchase.
There is no profit for them to use bitcoin, since most facebook users allready use paypal, it would make much more sence to integrate paypal payments.
Bitcoin is still not trustworthy enough, and its not that popular either, so people would have to go and buy bitcoin first, and then pay with it, which means they would have to loose twice as much time to purchase something.