@hugo around 20 years ago various large IRC channels were shut down because they were being used for C&C. I then wrote a test program that made Twitter usable as a C&C frontend. At that time there was still an RSS feed and you could easily search it for commands. I then steganographically hid the commands in cat images that were delivered as PNG.
Since I got the hang of it with my little knowledge and in a few days, I'm sure that such or similar methods are probably widespread.