I don't understand why people use the Facebook native app at all. Their mobile website works quite well and should avoid many of the issues with the app (location sharing, battery drain, etc).
And, given privacy issues and anecdotal reports of battery drain, I'll just forgo messages on that platform if they make the mobile site change permanent.
That said, messages are still working for me in Metal (third party FB app) and in my mobile browser. Even if/when they stop working that way, I figure I'll be able to tick the "desktop mode" box in my mobile browser to set the user agent and get it that way. Barring that I can just check those handful of messages I get via my desktop computer.
I guess I just don't depend on FB messaging for anything important so it's less an issue for me. If someone wants to reach me, they can call or text and Hangouts works fine across mobile and desktop on Android, iOS, Windows, OSX, Linux, etc. for group messaging.
They've also recently added WebNotifications support to the website messenger functionality which leverages the phone notifications as an app would.
When will these fine-grained permissions come to Android?
I had to borrow someone's phone 2 days ago to send a quick FB message and it took about 10x longer than it should due to it trying to hide the native interface "it's going away soon, install messenger".
Only by visiting my friends profile and hitting the message link could I actually break out. It'd still take me to the app store but on close my friend's messages were visible along with the box to send a message.
https://itunes.apple.com/us/app/paper-stories-from-facebook/...
The thing with Facebook is that I use it when I know I want to give away information. For example, when I'm at an airport about to takeoff to an unfamiliar city but I'm not sure which of my casual friends (too casual to send out an email blast) are at the destination, I appreciate the location-based feature of Facebook that detects that you're at an airport and makes it easy to autofill origin and destination. I want my location to be known and so, I might as well use the native app.
People use it because some people they like to follow aren't on Twitter
People use it to communicate with people who aren't on [insert competing messaging platform] because it's easier than convincing every friend to move to your preferred one.
People use it to track events because it has wider adoption than Eventbrite/etc. (and that platform choice lies largely with the organiser rather than attendees)
etc.