I don't understand why facebook isn't better at determining the intent of the message: hide messages with "following" "f" or whatever and just subscribe the user to notifications—which they could have done otherwise if the option was made more obvious.

Maybe I don't use FB enough but I hadn't heard of this "f" thing (beyond 'F' being a Call of Duty meme). I imagine it's an unexpected usecase for Facebook and one that would better be served by making a prominent "subscribe to notification" feature, rather than coming up with a machine learning solution. What if the parent comment is a quiz type question for which there are answers A through F? Admittedly it's an edge case but it's an example of how diverse the strange ways people use FB for commenting and discussing.