Ok To Add Your Teacher on Facebook?

So I came across a news story this morning talking about if it's okay for students to add their teacher on facebook as a friend...  This really brings me to lots of questions in general.  Let's think about facebook for a second.

Okay, seems like almost everyone is on facebook these days.  Some people seem to share every moment of their waking life on it, while others seem to be more limited, and then there are those who log in once a month randomly.

Personally, I would never put anything that personal on a website that anyone could see, but some people do. Teachers, probably should do the same.  What do You think?