An ex of mine, head of year 10 at a quite posh grammar school no less by her early 30's, was still VERY much the life and soul of every party within a 10 mile radius at the weekends...
Most of her Facebook photos were of her exceptionally drunk, doing stupid things, making lewd gestures (cigar in the mouth with her tongue puffing her cheek out being one for instance!) and generally being everything you'd hope your kid's teacher isn't!
Now don't get me wrong, she's an awesome teacher, she's calmed down and got married now too, but she was always known as a party girl and needed the weekends to release and be stupid and share it with her friends! Come Monday morning she'd have switched back into full on professional mode, where all her pupils loved her and she was very well respected by her colleagues... I'm sure if she'd allowed even one of her pupils or colleagues access to her Facebook page though, word would quickly have got out, and compromised the whole situation!
As has been said above... If you REALLY need to contact her at times, a mobile phone number or an email address will be fine. Sadly, everyone judges others by the content of their Facebook pages, so it's not wise really!