The New Oxford American Dictionary chose the verb "unfriend" as its 2009 Word of the Year (WOTY) and defined it this way: "to remove someone as a ‘friend' on a social networking site such as Facebook." The word "has both currency and potential longevity," explained Christine Lindberg, Oxford's senior lexicographer on the OUP Blog.
The choice of this year's word is telling because the act of unfriending (or defriending) is part of the normal pruning process of maintaining a presence on social media like Facebook, MySpace and LinkedIn. It's easy to collect more friends than you want or need, including many contacts that may turn out not to be "friends" by any reasonable definition of the word.
Fortunately, if someone posts too often, bores you, lurks without posting, has questionable politics or ethics, says something caustic or insensitive, acts unpredictably, or even uses too many exclamation points, it's relatively easy to get rid of them electronically---with no more than a few keystrokes.