I have no idea why you are downvoted to hell. You are correct, people do use F and other placeholder marks because they don’t know what the follow post button does on Facebook. I’m pretty sure OP meant the press f to pay respects thing but there’s no reason you should have been downvoted.
https://www.reddit.com/r/NoStupidQuestions/comments/8hn7x0/why_do_people_comment_f_in_the_comments/

