David Spark, a new media consultant and producer, interviewed me a few evenings ago on the awkwardness of social network defriending (e.g, taking someone off your friends list on Facebook, Linked In, MySpace, or Twitter). Here is the link to David's piece called The Awkwardness of De-friending. (You may notice that the jury is still out on whether defriending is hyphenated.)
Since there are no commonly accepted rules on the etiquette of how to go about ending face-to-face friendships, imagine how murky the rules of behavior are in defriending in cyberspace. The act of defriending is as easy as hitting a key but your decision can have long-lasting repercussions, both for you and the person you defriend.
My advice: Before you defriend someone, face-to-face or in cyberspace, take time to think before you act. Depending on the nature of your relationship, social media defriending can be the emotional equivalent of being jilted or jilting someone else. If the friendship was once meaningful and you change your mind after you've defriended someone, your relationship will never be the same. Don't let your fingers work more quickly than your mind.
David also wrote a piece published on Mashable, 12 Great Tales of De-friending and another on his own blog When technology tells us we have no friends. You may want to take a look at one of my earlier blog entries too, Online friending and defriending patterns.