There is no guarantee facebook is thinking about accepting bitcoin. They already accepted whatsapp and instagram. Right now, it seems like Facebook is getting ready for another new technology by accepting bitcoin. The company is ready to provide financial services and electronic money.
It isn't so much that facebook "accepted" instagram and whatsapp but they rather bought them out.
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.