Wearing a useless mask or one that's fitted / worn wrong is a meaningless gesture on your part. Since a good portion of those wearing one are doing that, what's the point?
Me I've been professionally trained and fitted for gas masks, respirators (half-mask type), and had OSHA and NAVOSH training on wearing those and even dust masks (yes, there is a right way to do it).
So, seeing some smuck in a mask with a full beard, or some lady that has her nose exposed, or an idiot with one that has big gaps around the sides and top, I just shrug and wish they issued signs...