I wasn't friends with any of my teachers on facebook until after I graduated. I was friends with the campus minister on facebook, but there was a much more personal relationship there than with any teachers.
And that is you. I pre-date facebook but I had a chemistry teacher that I would go to coffee with. I valued her opinion and I just liked talking to her. I didn't have a minister who I would share "boo" with.
I originally joined facebook for my cub scouts. Scouts and parents all shared pictures, ideas, etc. Every situation is different.
What gets me is as soon as someone says something is "for the children" all logic goes out the window.