Well, ever since the first YTPs came out, they were using memes and jokes that originated on image boards, 4chan, namely. I know ChickenPika posted here and that he's a chan user, but I really can say about many other poopers. I'd suspect that a larger portion of them are chan users than redditors though, judging by their attitudes and the content of their videos.
That's about the best answer I can provide.