I'm going to agree with the Steven Crowder pic/ bill board. I've heard lots and lots of talk about Flat Earth folks. But I seriously doubt they are a real serious movement to contend with. I'm guessing they are maybe a few thousand folks in the US.
How many people have ever met one of these people in real life? I can't say I ever have. I've met a few religious sub cultures like the Amish, but never someone like this.
The Internet seems to be force multiplier for strangeness. Fringe groups are much more visible or noticeable than they would normally be, even though to some degree the advent of Television, radio, newspapers did something similar in earlier years. e.g. Anton Lavey's Church of Satan got much more attention in the late 1960s to mid 1970s than it would have if it was started decades or a century earlier.