Similar to the concept of defriending, defraternization occurs when a person removes a friend or a sibling from their list of family members on their facebook profile. It usually occurs when a person decides to finally get 'serious' about facebook or they no longer regard the other person as a brother/sister.
Ex 1. ARJUN: Dude can you believe Aparna just defraternized me from facebook? She said something about how she wants to only keep her real siblings on facebook. Lame!
Ex 2. JEFF: Man this guy Pete is really annoying and I don't consider him my bro anymore. I'm gonna defraternize him.