One of the screenwriting sites I frequent had an amateur-written script called The Slender Man. His logline interested me so I read it, then went to the forums to discuss it. Only then did I realize, thanks to other members who had already responded, that The Slender Man was a deliberately created urban legend.
Personally I think it's horseshit. It's one thing to dream up some fictional boogey man if, say, it's for a movie, like they did with The Blair Witch Project. But for some doofuses to have so much time on their hands that they felt compelled out of sheer boredom to dream up some fictional boogey man just to fuck with a few people online? Get a life, already.