Re: I wonder how Facebook is going to implement the encryption
If its per-account, then there'd have to be some kind of infrastructure to allow the key to be copied between devices, at which point Facebook could incept it, if they aren't hosting the key itself.
I use Viber on tablet/laptop. When I've wanted to add a new device it's given a QR-code to link the account to the new device. One device acts as an overall 'master' or 'primary'
After adding a new device there's an option to sync data between them, which is not automatic.
It'd be quite feasible to have the devices use separate keys and talk to each other direct