A keybanger is a person that has limited success with their sexual escapades in real life, and therefore takes out their sexual frustration online, via trolling, tearing successful people down, or otherwise critical posts. They more vehement their postings, the more virginal they are likely to be.
Non-virgin1 - These kids on 4chan are pretty pathetic, eh?
Non-virgin2 - Yeah, these keybangers should get out of the house and try to bang a girl instead of banging their keyboards all day.
Non-virgin1 - Oh well, more hot tail for us!