Often Facebook Cheating is undetectable by the person being cheated on because most often, the harlet is not friends with the cheatee. The cheating can go on undetected for months before a friend of the cheatee notices what's going on and tells her.
This can often result in a Facebook confrontation.
Girl 2: Marrisa? My ex-boyfreind was facebook cheating on me with her. He wrote on her wall more than mine!
Girl 1: That's lame man.