I just asked my wife how did she go by replying to someone on her Facebook messenger mobile. She opened it up, selected one of her groups, touch a person's thumbnail once and the reply button showed up at the bottom. Then I asked her what happens if you press and hold someone's thumbnail, and her reply was "I don't know, let me check", "nothing". But then again, we are old enough to remember a time when there were no cell phones, no email and no internet...lolIt's pretty standard UI practice these days