A little ink at Learning and Leading with Technology, ISTE's print and online magazine: I responded a while back to an email from L&L calling for responses to the prompt, "Should You 'Friend' Your Students on Social Networking Sites? I came in on the majority side, as you'll see if you click to read the page I've screengrabbed below, but with a caveat: When former students are adults, as in college age or older, and if they still have an interest in their old elementary teacher and what crazy cutting edge technology he's into these days, I happily do "friend" them.
I do not do so with my own elementary students (I teach K-4 computer) nor with my son's Middle School friends or High School kids. Once they're grown, more or less (and I've lived long enough to know the best of my friends are still growing up) they're on, in my book.
See http://www.learningandleading-digital.com/learning_leading/20090910/ and find the Reader's Respond page to see my anecdote. I'm thrilled and pleased that they accepted it for publication. More than anything, I'm glad to have new way to keep up longstanding meaningful relationships.