When you are bored or you've been away from Internet access or just whenever you log in to Facebook over and over even though pretty much nothing changes, but you just do it to a) have something to do b) make sure no one's magically done something cool. Called such because it's like when you're hungry or (more likely) bored and so you keep checking the fridge in the hopes that something new and exciting will appear, even though you know it's just week-old leftovers.
Dude 1: Hey, have you noticed Lou's been on Facebook
a lot today?
Dude 2: Yeah, he was camping all last week...must have the Facebook Fridge-Check Syndrome.