Here's one crossover: the Blair Witch Project was marketed as an Alternate Reality Game, as were many of the later low budget found-footage horrors. It turns out that the top-of-the-list creepypasta "Slender man", in the form of the "Marble Hornets" youtube series, was mistaken for an ARG by the members of Unfiction.

https://en.wikipedia.org/wiki/Alternate_reality_game