Very recently, Facebook started using avatar images to replace pages or people's names in Facebook chat. If you typed the name of one of your friends, their Facebook profile photo would show up instead.
Not long has this feature been available that people are using it for something else entirely, rage faces inside Facebook chat being one example.
Reddit has a whole thread
dedicated to this. It's relatively easy to use rage faces in chat, all you have to do is employ one of the codes above, including the double brackets, or the many others that have been made available.
The trick works because people have started creating fake accounts and added rage faces as profile photos. The number in the brackets are Facebook account IDs.
While hijacking the feature to create chat rage faces is creative, it may not last long or, at least, you can't depend on it since Facebook could remove the accounts used for the images at any point and probably will.