Unless they radically change how Facebook messenger works I don't see how it could even be called end-to-end encryption. For it to work how it does now (online and independently through the app) they would have to hold the encryption keys.

They would have to instead tie Facebook messenger to a phone like Whatsapp does and use a web app to send messages directly from the device instead. I don't see how else it could be done and still be called end-to-end encryption.