When you like (almost) everything on someone's Facebook account. Usually caused by having a crush on someone and deciding that the best way to show your appreciation for him or her is to like everything that he or she has ever said, uploaded, or did. The most commonly projectile liked things are the profile pictures, followed by statuses.
A: Did you see C's Facebook account? D has liked everything he has ever posted or made his profile picture.
B: Yeah, D's into C as of the moment and somehow she thinks that projectile likes are how someone should show their feelings... by way of Facebook likes.