Face pro quo
You comment or like a post just to make your friend do the same. It's like you're both in a stupid competition. No one wants to be the last one to look like an idiot.
'That post is amazing!' (while your post is 'I’m alive')
'You look like a god!' (because you're wearing pajamas)
'That photo is legendary!' (while your photo is just your face in a toaster)