The deliberate deletion of a defined group of people from your facebook friends list. This can for instance be all friends acquainted through an ex-boy/girl-friend, or something as simple as removing those who you do not consider friends in your real life. As with a typical genocide, vulgar terms such as "cleaning up", "cleansing" or "taking out the trash" is perfectly in order.
Peter: I am so through with Mary and all her stoner friends. Yesterday I deleted all of them from facebook.
Alan: Yeah, a little facocide usually does the trick.
Christina: Based on the evidence we have seen from observations of his friends list, we have every reason to believe that acts of facocide have occurred after their breakup.
Allen: How many acts of facocide does it take to make a facocide?
Christina: Allen, that's just not a question that I'm in a position to answer.