Facebook Messenger for iOS and Android have long since supported the use of Stickers, and as of today, those using the Web version of the integrated instant messaging system should have no problem accentuating their text with one of those annoying, yet also rather endearing little faces. Although Stickers aren't something everybody is going to use, they do offer a little bit of fun and harmless immaturity, and having been successfully trialed on the two major mobile operating systems, Zuck and his team has rolled the feature out across the board.
You can level all kinds of legitimate complaints at Facebook, but not iterating is certainly not one of them. The Facebook web interface is currently undergoing some big changes, with those changes being rolled out to users on a staggered basis. We've not got all of them ourselves, but one that we do have access to is Stickers.