Let's see, public school teachers (I was one for 5 years) are expected to spend 50-70 hours a week perfecting their craft and spend 'extra time' tutoring/helping/counseling students. On the plus side, I'm sure thousands of students over the years have been helped by a caring and nurturing teacher; I'm not sure I would've survived high school if it wasn't for a caring English teacher.
However, is it any wonder that teachers feel that the only human contact they get is the children they serve? And when a group of students drag you into their 'drama' (and it happens quite often), it is easy to lose site of your real goal, which is to teach a subject.
Facebook (and other social media, including texts) is just another vehicle to be abused by the side effect of 50+ hours a week with students. Would you, as a teacher, give a student and his/her friends a ride home in the rain? How about some $ for lunch? Advice? What if they call you at home? Of course, there are proper responses to these questions, but the lines get blurred very quickly.
The Missouri law is just a response to this. Popular teachers are often the ones that blur the lines of professional separation. And, in a professional sense, a teacher is often criticized if they are not 'effective'--and popularity is often a side effect of this.
Common sense says a teacher shouldn't friend a student. Professional sense says you should be available to your students. Legal sense says you should steer clear (as I often did). But after 50+ hours a week with teenagers, what time/energy do you have for other nurturing human contact (and often teenagers are very nurturing, sometimes for the wrong reasons admittedly). AND, in many states, getting caught (by photograph but often by a parent) at a bar/party/unpopular political event (including religious) will cost you a fine, a reprimand, your job, and even your license. I like live music--do you know how many times I and my teacher friends had to make sure the club/bar owned did NOT take our pictures for his popularity wall? So where else does a teacher go for 'real world' contact?
(flame shield on)--Obviously, there are things teachers should/could do to get positive relationships. BUT, we are talking about people who, in a general sense, are like you and I: We know what we should do but get away with blurring the lines because we are too tired/angry/desperate to 'follow the rules'-- (flame shield off).