I've been giving my situation a lot of thought. And one thing has occurred to me.
There seems to be a certain type of mind that attracts the Slender Man. A specific lack of mental immune system, that lets him into your head.
Everyone started with one little breadcrumb. The EMH kids, Marble Hornets, Seeking Truth, White Elephants, Just Another Fool. Gateways. Not just into the mythos. Gateways for him. The Slender Man is as much a mental entity as a physical. You don't see him unless you know. Children of course, are willing to accept the implausibly tall, faceless man in a suit.
Once you find that first little bit, once you're in the know? You open a door for him, just a crack. The more you learn, the more that door opens, until eventually you step smack-dab into Mr. Trim and Slim. And there's another problem. Curiosity. The minds that seem to find the Slender Man all possess it in one measure or another. Detectives, journalists. It just takes a little step, once you find that first bit, you're drawn to the next breadcrumb like iron fillings to a lodestone.
Picture a forest. A dark forest, filled with trees with black leaves. You walk into the forest one day, only to find that you can't remember your way out. You could pick a direction out at random, but all you do is lead yourself deeper into the forest. At in the forest lives a terrible monster. If you try to find your way out of the forest, you run the risk of running right into the monster. Or you could simply stay away from the monster and hope you find the outside edge.
It's paradoxical. You need to know more about him to survive. In fact, it's a drive to understand that will probably kill most of us in the end. The more you learn about him, the more you open that door up. Why do you think Strahm, who was once brave enough to try to shoot the Slender Man, has become what he is? Knowledge. Why is it that the bright ones always burn out, or worse, turn proxy of their own volition. The brightest, those with the most knowledge, are snuffed out. It's all about the knowledge.
It's why people like M, or Spencer and Co. will continue to survive. It's why you don't get better. It's why there's no way out of the forest. And the storm is coming.