As a TA, how to react if I come across a Facebook group in which students insult me? I am a PhD student in Europe and also TA for some bachelor's degree classes for my supervisor.
Just by chance, I have found an open Facebook group (normally, the group aims to create an interaction between students and opened by students as well.) in which people in class insult me very harshly about the way I do the tutorial and my personality. (How I look etc.)

How to react or not to react, really? Any suggestions in order to be objective in evaluation of those students?  Because, I don't want to introduce any personal judgemental issue for those who insult when I grade midterms.
Sometimes students behave childishly. They've only been adults for a few years, and some of them are still transitioning. In groups in particular, and with the anonymizing effect of the internet, immaturity can be amplified.

The first thing to do, when faced with childish behavior, is increase the maturity of your own. Show them how childish they are by contrast. In this case, the most mature thing to do would probably be to take the valid parts of their criticism on board, and ignore the rest. If you feel you must mention it, reduce your comments to the absolute most reasonable and objective part of what you feel needs to be said: you didn't enjoy reading this about yourself, they should come to you directly (or go to your supervisor) with valid criticism, and they should consider the nature of internet posts for the sake of their own professional career.

Don't put the comments on the projector, don't single people out and don't open up a discussion. Just mention it as briefly as you can, and move on.

If it helps, consider that you are in a position of authority, and this is what happens to people in authority. Any boss or manager in any company will have to deal with the fact that people will have little bitching sessions behind their back, and usually not very mature ones. The higher up you get, the more people will gossip about you. There's no way to escape it: no matter how cool or normal you are, the tiniest idiosyncrasies will be magnified.

It's usually not meant in a bad spirit, just a way to blow off steam about forced circumstances. The problem here is that they did it on the internet, instead of in the coffee room.