After his introduction on Something Awful, Slender Man started making regular appearances on other message boards dedicated to horror, specifically the 4Chan board /x/ paranormal. On boards like this, and other places such as the Reddit board /r/nosleep there is an entire culture of horror on the Internet designed around the idea of fictional stories treated like reality.
Much like how wrestlers try not to break “kayfabe” and recognize that wrestling is fake — despite the entire audience knowing it is — so too do online “real-life” horror writers treat every story as “real” despite the fact that most of the audience knows they probably aren’t. Within these communities, Slender Man became a recurring memetic device and character.