I am surprised by the popularity of what could have been a simple image for a contest. It didn't even remain a simple Internet myth: it embedded itself in modern media (two examples I know of are My Little Pony and Lost Girl), even before Wisconsin's Slenderman Stabbing (which I'm still not sure of its impact on medias view upon Creepypastas communities). I should try researching on the supposed folklore origins of the Slenderman (this "Großman" or the "Richter"), but I'm afraid I'll be drowned in fake conspiracy theories.
I'm not talking about taking them "seriously", but whether they are even creepy as stories. There is only one time where the story has been applied in a way I really like.