Tagversation
When there is a photo on Facebook with numerous people tagged, and two or more people decide to have a conversation in the comment box, sending an email to everyone tagged for each message they post. Easily avoidable by changing preferences, but nobody has the patience to do that. Also caused by someone that has gone commental.
Man, Jake and Andrew had a tagversation on our group shot, and I got 40 emails the next day!