The only thing smarter than the viral marketing campaign is the indictment of society at large...
1) People used to believe that other people were witches, and would kill them accordingly. This was called "The Burning Times."
2) We have seemingly evolved in the last few hundred years, and don't do that anymore, at least not in a "mass genocide" sort of way, and not because of witchcraft.
3) People are surprisingly ready to believe that a real witch, whose nickname is Blair Witch, but whose real name is Elly Kedward, killed three real college students, and possibly more victims. People believe that three college students went back in time about 50 years, were killed by a witch and / or her servants, and then the footage of their documentary was buried, and the footage was found about 51 years after it was buried. There's a documentary and a website as proof.
4) People invade the town that they see in the movie, called Burkittsville, and search for the witch, or any signs of the witch, or the missing students. The website crashes from so many visits.
5) The directors explain that the movie is fake and that the witch, and the lore surrounding the witch, was entirely made up by the directors. Society still thinks that the Blair Witch is real. Some keen eyes notice that certain items in Rustin Parr's house were made a good decade after it supposedly burned down. Other people notice that there aren't enough woodsy areas in Burkittsville for anyone to get lost in, and that some of the movie was probably filmed in nearby Seneca Creek State Park, a state park that looks drastically different than what the unkempt woods would have looked like 50 years ago. Society demands that the Blair Witch is real and that authorities do more to find the bodies of the poor missing students, and bring to justice the 200 year old Elly Kedward and her wicked satanic servants. Invaders of Burkittsville cause chaos and destroy personal property. Violence ensues. The actors have to actively come out and explain that they are still alive, simply actors, and how the film-making process worked (GPS, ad-libbing, method acting, letters written by directors for the actors to find, etcetera) before people believe that it's fake.
6) Audiences now view The Blair Witch Project
as a standard horror mockumentary, with actors performing, essentially, a hoax... for money... and there is nothing supernatural about it, but audiences still believe, in A True Story In My Universe
sort of way, that a fictional witch killed three fictional college students, always wondering why the witch is never seen (as seeing the witch would make it a better movie). Viewers believe that Elly Kedward was a child-murdering witch who got what she deserved, despite no evidence to her being a witch, and no evidence pointing to anything other than that she was falsely accused and unjustly murdered, like what happened in the Burning Times.
7) People barely remember The Blair Witch Project now, let alone apply their experience to other "photographic / video evidence" of the supernatural.