Before when The Blair Witch Project was released, the marketing for the film was quite unique at the time. The hype surrounding the film was that it was real and based off a true story. It was indeed marketed as a real documentary and was one of the first found footage films out there.
The producers even went as far as creating a website with additional photos, "evidence", bios of the filmmakers and more to help with the realism. A TV special mockumentary entitled "Curse of the Blair Witch" was also released which followed the "investigation into the disappearance of the three film makers."
Also fliers were distributed making it seem like the three actors were indeed really missing:
"During screenings, the filmmakers made advertising efforts to promulgate the events in the film as factual, including the distribution of flyers at festivals such as the Sundance Film Festival, asking viewers to come forward with any information about the "missing" students." Source
Many people fell for the marketing ploy that the Blair Witch was real, and back then the internet was not as prominent as it is in today's world for finding out information. But even the IMDb page for the three actors stated they were missing and presumed dead.
People also really did come to Burkitsville:
"Burkittsville was swarmed. Cars and tour buses jammed Main Street. Residents couldn't get into their driveways. Souvenir hunters dug up cemetery dirt. Tombstones were vandalized. Kids, accustomed to riding their bikes with no hands down the farm alleys, were instructed never to play outside alone.
Debby Burgoyne, the current mayor and a Girl Scout leader, found a strange man standing in her living room one morning. He thought there was a tour.
"It was crazy," Burgoyne said. "People with cameras were everywhere. I made sure I had full makeup and a great nightie before I went out to get the morning paper.""