When somebody mentions one of their friends on their facebook page praising them or saying how good their friendship is, then for no reason just deletes it, insulting and upsetting their friend. Consequently something bad happens to them or they get bad luck. A cyber version of "what goes around comes around".
Girl: That boy that I've fancied for ages told me he didn't want to go out with me.
Girl's friend: Well you know why that was, because you replaced the thing in the little box on your facebook saying "Kate and Jenny BFFs" for a film quote. It's facebook karma!
The kick you get when something you did, wrote, achieved, suffered, posted on FaceBook earns you that "47 people liked this" note above your message.
Bill was surprised that 148 people sent him get-well messages when he broke his leg. Talk about FaceBook karma!