Just a guess, though. I don't actually FB.
Facebook is a weird place if you don't have an account. And of course, whenever you scroll down for some details (and hopefully a link to that business' proper website) you get slammed in the face with a dialogue box that asks you to create an account or login, so my engagement with Facebook usually ends there.
It's grown to be a more common way to share something with a friend. Less clicks than sharing in private messages, and also more inviting to conversation — which would be shared not only between you two, but also open to other friends like yours. Mostly those @mentions and conversations are very casual, like "me: @friendName -- @friendName: OMG this is so @frank -- frank: I'm in this picture and I don't like it". All of which makes perfect sense for mindless positive chit-chat.
So if one of your friends comments with the one millionth comment, you can end up seeing the post and your friend’s comment In your feed. So while no one can read all comments - your friends are likely to see yours.
(Perhaps once upon a time I'll end up also finally figuring out how the hell comments on Tumblr work. Not that I'll have practical use for that anymore.)
It works in similar way as here. The post have 145 comments (right now) and yet I'm adding another one.
Almost always when I find myself in Reddit comments because something's interesting I wish it were HN, and am shocked a lot of the time that people have time for such drivel. Duolingo too. Yes. Me too. Haha. Nice work buddy. Looks good. No. This is lame. Lol. Why? What's the point?
But then, we can take this further, there's more thought in our comments, and on HN in general, but ultimately not any 'point' either. (Obviously there's the occasional hugely noteworthy comment on HN, but then I'm sure there is on Reddit too, albeit fewer pro rata.)