George Couros has a thoughtful essay about how the personal vs. professional debate as it concerns teachers and social networking might be better reframed as public vs. private.
For example. let’s say a student wrote about how much they hated another student and started bullying them online. Does it matter if the student said, “well this is my personal account”? Even if the student wrote it in a “private” email, it can become public with a quick screen capture and shared with the world. To me, anything that is posted online, you should consider “public” no matter what your “privacy” settings are.
The endless Facebook debates seem to have died down at my school, but everywhere I go Facebook is a hot topic of conversation. My general rule of thumb is to not post anything on Facebook I wouldn't want my mother to see. And since my mom is one of my Facebook friends this rule is grounded in reality.