Facebook replaces its old inbox with the web version of Messenger. It can sound like not a big deal for you, but it really is and let me explain why. Before that Facebook had 2 versions of messaging UI: old crappy one without support of all the bot features (quick replies, location and many more) and the second one which was a Messenger UI - with support of all the features you need to get the full experience of bots. And now Facebook redirects you directly to the Messenger when you want to open your inbox. It’s a huge step to the unification of conversational experience for Facebook users and minus 100XP points of pain for all the bot developers.
https://techcrunch.com/2017/01/18/facebook-replaces-its-old-inbox-with-the-web-version-of-messenger/