This also happens in sports. Someone says something in an interview, in a social network post, in a private conversation which gets published... and what happens? Depending on offense the team may, ignore, fine, suspend or fire the person in question, outside the legal framework. That is, someone may say something insensitive or can be accused (not yet proven) to have done something unbecoming... teams often feel pressure to act extrajudicially. It's just how it is where money and the public sphere of opinion intersect.
Moderators do refer the most serious cases to the police. But there's a lot of crap that falls short of that, so milder penalties work better.
You actually understood me very well. That's my secret point. I don't see why FB must do better than police. If someone insults me, then, well, I fight back, go to the police, prosecute, etc. That works well because there is the work of justice.
By displacing the problem out of FB into the realm of justice, I want to make two points :
1/ The social norm of what is acceptable or not acceptable speech is governed by the people, not by a single entity.
2/ If there are too many cases to handle for the judge, then, let's add more judges (in my country, justice department is starving for budgets).
FB has way too much power, that's its (totally fair) success but now, it's too big to be let alone.
Let's imagine that there is no moderation. What will happen ? OK, some people will have argument, there will be insult, blood, etc. And, well, depending on a tipping point, we may realize that oooooops, letting people talk in the open is not so good. FB would be blamed and hen disappear. So be it. Now, another thing to do is : prosecute those who say unacceptable things. With prosecution, you have the justice system that will handle the case. That'll be slower but there will be discussions and, hopefully better laws.
FaceBook is becoming the Judge Dredd of free speech. It decides what's criminal and what's not and it applies his own power to handle the case directly.