I really don't understand tagging friends in a comment on FB and IG. This is precisely what the share feature is for!
Sharing to someone else's timeline is way too invasive, sharing to your own timeline and tagging the person is weird, sharing via message is too disruptive.
What people is looking for is a "Show it to a friend" button that just creates a notification on the person's bell icon and allows them to respond in any way they want.
Are shares not as good of a metric as comments (even though most comments are effectively shares, making them even more noisy)?
Also, basic behavior mimicry. You see other people do it so you also do it.