At the same time, the app can be set to make messages disappear after a certain amount of time. Facebook says that this new feature is based on the Signal Protocol developed by Open Whisper Systems (PDF link).
There are differences when using FB’s Secret Conversations: at the moment, some features such as video, payment or animated GIFs are not supported, possibly because the messages are relayed by a different infrastructure which needs to be ramped up, or to study the new functionality before adding features slowly.
There’s a good chance that E2EE will eventually make its way to the general messaging, and probably to all aspects of Facebook’s chat, but it will take time, and would happen in the background.
At the same time, Facebook is catching up with some of the features that snapchat made popular. With 900 Million monthly users, this is not something to be underestimated, and although there are many popular chat apps in the world (including WeChat, Kakao, and Line), Facebook is getting serious about competing on every turf.