It's Imgur because the upvote turns the heart green.
Imgur comments 2014: http://imgur.com/gallery/4DQZTf2
Imgur comments 2015: http://imgur.com/gallery/UaKGHvQ
Green Heart is the new Imgur upvote:
Stashed in: gifs, G4!, Imgur!, Upvote! Downvote!
Original without the heart turning green:
From a collection of Katy Perry gifs: http://imgur.com/a/7I1pd
There are a lot of gifs on that page, actually.
Imgur comments: http://imgur.com/gallery/irwduR9