Part of the purpose of the 'like' button is obviously help form a more complete profile of a given user in order to more effectively customize advertising - would incorporating a 'dislike' button make this task any easier? That is, would an approach that refines advertising content by filtering out subject matter a user actively dislikes have a substantive advantage over an approach that just focuses on homing in on what the user actively likes? My guess is that it doesn't, otherwise it would have been implemented already.