Face pro quo
You comment or like a post just so your friend will do the same. It's like you're both trying to out-sucky each other. No one wants to be the last one to look bad.
'That photo is legendary!' (while your photo is just your face in a toaster)
'You look like a king!' (because you're wearing a shirt from 2004)
'That post is amazing!' (while your post is just 'I'm alive')