Someone who posts something random on your Facebook wall, or comments on a post for the sole purpose of being seen by someone else/making their presence known. Usually done by an ex-gf/bf, a wanna-be gf/bf or in sneaky cases, by someone who wants you to friend them but does not want to send a friend request for fear of being rejected, so posts it on mutual friend's page.
Usually this pop-up is ignored, like most screen pop-ups because it's an obvious "look at me" attempt.
Jenny: "Did you see that random post on Tom's wall from his ex?"
Alexis: "Yeah, I'm not worried. It's just a wall-stalk pop-up. He doesn't even talk to her anymore."