"The Slender Man" is an ongoing interactive horror project that has the unusual distinction of being truly open source. Anyone can contribute to the mythos, and the overall canonicity of new material depends on how much of the community embraces it. You can get a feel for how bizarre the results of that approach are by paying a visit to the Slender Man Wiki.
One of the things I find fascinating about the Slender Man community is how much it mirrors the UFO subculture. Both incorporate certain landmark events into the overall narrative (i.e. Roswell crash, Stirling library fire) and then add layer upon layer of interpretation, casting them into a new light. Inevitably, that leads to the development of cliques that go to war with each other, which in turn produces charges that outside agents are intentionally spreading disinformation. What's amazing is that it still happens when everyone knows it's all fake to begin with. Whether that's a sign of creative brilliance on the part of Slender Man's community, or an indictment of human nature, is open to interpretation.
Andrew Wishart brings us these artifacts that are sure to add to the debate. Where they came from is a bit sketchy, but their existence would seem to indicate that there is indeed a cult or group of followers devoted to Slender Man.