The monocle itself is not so much worn as it is held in place by the wearer's facial muscles. The monocle is sized such that it can be held in place between the wearer's eyebrow and upper cheek by a slight squint. Because of this, the monocle could easily fall off with the wrong facial expression.
Usage of a monocle is generally associated with rich upper-class men, as well as uptight individuals. Combined with with a tuxedo and top-hat, it completes the costume of a stereotypical gentleman.