Well there's some pretty amusing abuse. I recall there was a botnet a while back that got it's commands from a subreddit as well. Quite brilliant actually- who would suspect reddit as a command server?
Everyone who used IRC as a command server from years past. It turns out that things useful for human communication tend to be useful for computer communication.
Usenet, Email... hell... I'm sure BBS would have been used if modems were popular enough back in the day.
encode your info as bits in the image ;)