The app I use is actually "Facebook Lite" and it does include Messenger.

>maybe you're using it for a purpose that doesn't require instant notifications.

Yeah, generally if I'm messaging in Facebook, it's someone I don't know well - often kind of temporary connection through a community group, like arranging a time to exchange a second hand thing. I'm happy to manually check when needed. (That is, back when I used to use this method before I gave up and got the app.)

This comment does give a bit of insight into why Facebook tries to prevent people using mobile browsers for messaging though.

I was aware of Facebook Lite and had it installed at some point when I was more willing to use Facebook. I (probably wrongly) remember it not having messaging, because there is or was a separate Messenger Lite application.