Wearing my mom hat, I finally allowed my 15-year-old to get a Facebook. I've kept her off MySpace because, well, I just didn't like the stuff I saw. But she returned from camp and wanted to stay current so she's there. And I have a MySpace and Facebook and so far so good. But we've had many talks about the public nature of it all (she says that's why she likes Facebook - the h.s. pages are just for h.s. students.)
That hat off, I think we have to continually raise these questions and encourage students to think about ramifications of a public shared life as you say aptly above. And phase in their use of this public life so they understand what playing in a place where the whole world is watching really means.
Yes, certainly the conversations I am having with my kids about the uses of technology (when and how to use) are much more sophisticated than we were having five years ago! My two oldest (17 & 19) have Facebook, and I'm their friend. My two youngest use a Ning network I set up for them and their friends (which, btw, they LOVE--isn't that interesting...).