It is already possible to pay for things via bitcoin on facebook and IMO it is only a matter of time that facebook will start to accept bitcoin for payment for ads and likely credits. Also since it is possible to purchase many gift cards on this forum, I would not be surprised to start to see trading of facebook credits on the marketplace sometime in the future.
Facebook credits will slowly fade away. Merchants might decided to by pass payment system and accept and deal using bitcoin directly.
Off platform transaction occur on ebay and amazon all the times.
The only issue is trust. For facebook store with reputation, they can easily convince buyer to send payment first before shipping items.