There are three very distinct separate issues here.
1.) All employers should have very clear guidelines, clearly disseminated about what they consider your private live and what they do not. As an employee you relinquish many of your rights. People need to be aware of that and have the boundaries explained to them.
2.) Teachers used to have to live in the schoolhouse and were watched by everyone. There was a time and place when this was the norm. It is not now, nor should it be. Within the confines of the above a teacher's private time is just that - private. Assuming s/he is not with students, or recounting her/his exploits to students, if it is legal, it is his/her business. (Geez Writer, some people need to get a life!?! Sorry that happened to you.)
3.) Facebook's very premise is to remove the boundaries in your life. The idea is to make all aspects of your life seamless. This is a very bad idea. People have boundaries for a reason. We all have the right to a public and a private self. None of us, not a one, can be on public behavior all of the time. This is the stuff that leads to Brittney Spears and Lindsey Lohan. No one can withstand that stress. The corporation that is facebook can and does change privacy settings on people without notice. And the settings they have are about as private as a first floor motel room with lace curtains.
We all have friends we don't want to have sit down with our parents. We all need to have the ability to grouse with our fiends about our bosses. But, now people don't see that facebook is NOT PRIVATE. No matter what you think your settings are. Do not ever put or say anything on facebook that you are not willing to see in a headline. Yes, it IS that bad. Today's young people do not understand this and it will haunt them for decades.