In the 20th century, with pop culture conditioning of movies from "holy-wood," this became associated with "A-okay" especially during the second world war.
The thumb makes a circle and the three up pointing fingers then make three successive sixes. Ergo, 666. You see famous people use this all the time, but not regular people, in the main.
They are signalling to those in the audience, readership, or media that they are one of the faithful to the dark side - and also as a way of instilling in all the credo they love to quote so much among themselves, "we are everywhere, everyone, and control everything."
That may be...for now.
But we see you. We know what you are. You are Billy's bitches and to us, it only means you have no will of your own and are just another order taking grunt in the army of darkness.
Big deal. Really. So what.