Facebook Mondays, or FBM, occurs when a facebook user enters their account on Monday morning to discover they have been tagged in numerous photos that capture their recent indiscretions of the weekend. Typically this includes photos of one engaging in activities that they're not particularly proud of. As a result, the individual finds themselves having to untag multiple photos to relieve themselves of embarrassment. When a user finds themselves in this position they have been hit with a case of "Facebook Mondays."
Susan opens her facebook account with a look of shock on her face.
Bob: What's wrong?
Susan: Facebook Mondays...there's so many drunk pictures of me from this weekend!
What?! I had no idea I was making out with that chick, who took these pictures?! FBM!