It's on Facebook, too. First noticed it a few months ago on a slow mobile connection. https://i.vgy.me/98RIOV.png

It's in the alt text of all photos.

It won't be long before all the Instagram bots are commenting 'nice person!' instead of just a thumbs up emoji.

Came across this yesterday as well. Was on a slow internet connection and the images failed to load. Pretty neat.