I found this interesting-- an infographic about the amateur horror stories (like Slenderman) which have circulated on the Internet. This offers a "recipe" for what makes a horror story viral. I remember-- long before the internet made it easy-- we used to circulate stories like this, telling them at Girl Scout Camp and at school, and they would make it across the country just one kid at a time. Everyone remembers the "Hitchhiker" story, for example.
"I decided to sit down and read 72 of the most shared creepypasta stories to try and figure out the recipe for a viral horror story. Check out my findings, along with a spooky infographic, on our blog (warning, some of the content is preeetty freaky):