No, we don't know if they are watching us at all, because we don't know what's on the "canvas". The difference is though, that when they watch they see exactly what we want them to see. The screen isn't the canvas until someone writes a comment or makes a follow-up post on something said. That's the true canvas. And then we truly know that we are watched. Like when you would turn the canvas around in the painting. But we cannot. We never know what's inside someone's head.

So commenting and writing follow-up posts breaks the uncertainty, because then we know for certain we are being watched. That's when the canvas gets turned around and we see what is there, what was hidden.