I tend to tag comments if they could be a standalone comment. So I wouldn’t tag a “good job” or “happy birthday” reply. (BTW, happy birthday.) But if I’m giving a substantive answer that can stand alone or make sense with the highlighted quote from the article I’m responding to, then I tag.
As for Instagram, I really only use it to feed pics into FB, so I’d defer to your nephew expert. And I verify that I skim posts with strings of hashtags.