I pretty much agree with Twisted Tree,
But I have another take on it.
I was neutral about FB until I became an educator.
A site that started as a "hook-up" site for college kids, is a disaster for younger kids.
As Peter said, it's addictive and particularly so for hormone driven kids. Thus it was MAIN source of bullying and conflict in my schools!
It did not help that a few teachers, very few, also used it in an unprofessional way.
Wifey B: Well, I have to tell you that unfortunately the educator issues with FB are now minor compared to Twitter and Instagram. There have been many hate messages aimed at teachers posted on twitter and many very cruel personal comments. Other's just vulgar references regarding teachers. While the internet has become a means of saying anything you want, anytime, about anyone, twitter has exemplified the worst extreme of that. I'm sure the comments were always made in small groups of kids but now that they're blasted into the world, it's very sad.
So, bullying of other students, of teachers, of others in general. One post, whether true or not about someone's sexual preference. One about a someone's appearance or their intelligence. Bullying is one thing to deal with face to face, but so much harder online and regardless of what you do the cat is already out of the bag. You can't undo the damage.
As to FB for younger children and all the other issues, it can be bothersome and dangerous, but the worst seems to have moved on. However, it's still a first contact point for much that moves into un-moderated chat and text.
And, yes, I've seen posts by teachers on FB about how awful their kids were today or how some were just bad kids or stressing them out. This is, in my opinion, even worse than posting hate toward your boss online. You say anything negative about the kids you teach online and you're on my s... list.
Now I think what we're talking about here are issues with social media in general and that's part of our lives now. Not a FB problem only. Not something that would stop me from using FB probably. Something though that just adds to my lack of love for it. You friend your colleagues and then learn things about them you wish you'd never learned. When you go to school the next day, do you confront or pretend you didn't read it? Rumor and innuendo do tremendous damage. At one time, they had to travel one person at a time. Now they can spread to hundreds in minutes.
It's part of the price we pay for the incredible world of the internet. I don't like paying it. If I had a compelling reason to be on FB I would be. And for any who think those of us not actively participating there don't know about it, you really underestimate us. We've viewed. We've thought about it. We've made informed choices.