There is a piece of the Slender Man mythos that is not immediately obvious. It accounts for the belief in something that is "obviously" made up. I thought at first it was just kids being "in character" with the genre, but it goes beyond that. Many of them REALLY do believe in Slender Man, think they have seen him and are certainly having nightmares about him. It is having a real impact on their lives.
Kids do know that Slender Man was made up in 2009. However, the mythos discusses the notion that Slender Man is a "Tulpa":
Tulpa - Wikipedia, the free encyclopedia.
Briefly, a Tulpa is a demon that can be brought into "existence" through belief. If people believe in it, then it is real. If you think about it, this is how every god, demon or other entity gained existence and acceptance. It doesn't matter that you cannot prove or disprove the existence of, say, Slender Man, in exactly the same way that you cannot prove or disprove the existence of (any) God. For those that believe, the Tulpa becomes "real". All that is required is a "seed", a picture and/or stories to which belief can be anchored and expanded upon.
I think this is an incredibly important aspect of Slender Man that has simply been overlooked (or ignored) by the media and the public. It goes long way to explain how someone susceptible could set aside common reason and do something as dreadful as attempting to sacrifice a friend. By "someone susceptible", I'm thinking about mental illness, loneliness, isolation, someone that has an important part of their life missing or damaged.
My fear is that the part that Slender Man has played in this affair will be simply dismissed, the girls will wind up in prison for the rest of their lives, and they won't get the psychiatric treatment they need. In the meantime, something like this will happen again, perhaps without the great good fortune of the victim surviving.
The following is a very interesting read about how Slender Man came about and evolved.
http://darklore.dailygrail.com/samples/DL6-IV.pdf