Unfortunately quite worthless if the client is not open source and still controlled by FB. Having e.g. analytics running in client will not improve privacy at all, no matter how the messages are encrypted. (Also the metadata can expose more than enough information)

I welcome this news, however admitting that it might have serious concerns. Facebook probably will still have some method for getting your messages if they need to. The promise of E2EE can be dangerous because a false sense of security is worse than acknowledging that something is insecure and acting accordingly.

How do we know they are really doing this? The app is closed source.