The naughty ones
may encrypt a plain text file and steg it into the picture of a kitty, then post it to Facebook, Flicker, Pinterest.
Alternatively they might post a paragraph of text to Facebook or a forum where every prime numbered/Fibonacci numbered character in the text spells a message. Perhaps this is not too bright, if I thought a message was hidden in plain text I would write code to pull any desired sequence of characters from that text. I am pretty sure GCHQ do that kind of thing now with suspect text.
That's off the top of my head without any thought at all, just like an MP's thought process.