The Fart Button
The Fart Button was added on the 13th November 2014 as a parody of Like buttons. The button is entirely a joke and causes absolutely nothing to happen, except for a fart sound that plays.
The Twitter Thread contains some reactions from the day The Fart Button was added.
The general procedure to use the button is to press it, and then reply to the post to tell them that you pressed it. The Fart Button appears on every post, even bad ones, so feel free to fart to your heart's delight.
In August of 2020, some nerd made a thread discussing a mysterious request that was made upon clicking the like button. The request was sent to an IFTTT hook with info on who farted the post, what the post said, and a link to the post. At some point the functionality stopped working due to the CORS policy blocking the request.
While this was supposedly just a feature to notify Rena when someone clicked the fart button, more skeptical users believe that a sizable cheque was cut by Jeff Bezos to add telemetry to the site, and use farts to improve product recommendations on Amazon.