While that is true, many times what person A considers to be harassment, the person that is harassing them does not consider it to be harassment at all.
This is not to say the person is not being harassed, although I have seen many examples of people claiming to have been "harassed" but when I view the twitter history for example I do not consider it to be harassment. That said often abusers do not self identify as abusers for example a Wife beater does not believe they are abusing their victim.
>>Facebook and Twitter struggle with this, partly because it's impractical to manually moderate a site with so many users and partly because their model doesn't really allow it.
Facebook and Twitter struggle with this because they refuse to create the clear rules for participation you stated other sites do. They proclaim to be "Free Speech" platforms welcome to "All Ideas and discussion" while creating mushy and lose rules that are highly subjective and makes enforcing those rules problematic.
Where as the sites you describe are normally subject matter sites so it is somewhat easier to define a more ridged rule set.
Facebook, twitter, and to a lesser extent Reddits problem is it impossible to be both a "Free Speech" platform and a censor... These are mutually exclusive concepts