That seems a sign of the times - unfriending people is a word that “has both currency and potential longevity", according to Christine Lindberg, Senior Lexicographer for the New Oxford American Dictionary. It's a pity the friendships don't also have longevity. I must admit to a cynical attitude about friendships maintained exclusively on Facebook without some personal interaction to bolster them. And perhaps we live in times when everything is disposable. Even people.
Most “un-” prefixed words are adjectives (unacceptable, unpleasant), and there are certainly some familiar “un-” verbs (uncap, unpack), but “unfriend” is different from the norm. It assumes a verb sense of “friend” that is really not used (at least not since maybe the 17th century!).I recently posted about Chad Taylor's use of the modern verb friend and I subsequently discovered a Grammarphobia Blog entry dating a similar verb, to friend, back to the thirteenth century.