Bacn is on purpose spelled wrongly and relates to Spam – and it indeed becomes a growing problem of my inbox … see the definition from http://lifehacker.com/software/ask-the-readers/how-do-you-handle-bacn-291688.php:

Email you receive that isn’t spam… And isn’t personal mail. It’s the middle class of email. It’s notifications of a new post to your Facebook wall or a new follower on Twitter. It’s the Google alert for your name and the newsletter from your favorite company.

See also the Bacn website – nice T-Shirt, with a shorter definition:

Email you want, but not right now.