How are these distinct from imageboards, forums or reddit? This is a very slippery slope which implies censorship from the very beginning. Are you prepared to follow your reasoning all the way to its conclusion and stop people from having any platform for speaking their mind? Once you go there, you no longer have freedom of speech, which I agree with Hitchens et al. has to be protected even when it is uncomfortable.
I agree that there is a very tricky ethical case when it comes to harrassment or terrorism, but I have no good ideas on how to resolve it. We should err on the side of not censoring stuff we don't like.