@dssf
They need not be purchased by real money, but are "checked out" so to speak from a collection of various packages of icons. No transfer of currency, although the entire process is set up where that wouldn't be difficult to achieve. I've thought that was the plan for the future since I first saw them. Change the price from "FREE" to "$.99". Also, they're only for the "Messenger" portion of the website or official mobile app.
In fact, they're VERY annoying when I get one using Pidgin/XMPP to talk to people who insist on Facebook. When the other party uses one, I get a message that has the URL to the conversation so I can direct my browser to facebook to see the inane little icon I was just sent. Always well worth my time, I assure you </sarcasm>