People in social media say many stupid things. "Exterminate the Jews," "Exterminate the Arabs," "Exterminate the blacks," "Exterminate the whites," "Exterminate the males," "Justin Bieber is awesome." Saying stupid things and doing stupid things are not equivalent, and no group should have special protection. If we should not be allowed to say "exterminate the Jews," we should not be allowed to say so of anyone else. Why single the Jews out like this?
Now, as far as free speech is concerned, this approaches the fine line. If a person is just making a logical (or illogical) proposition, an expression of his hatred or what have you, that is one thing. For example, "Well, if you want society to be X, then you're just going to have to exterminate all the Y." We might not like it, but that's free speech.
But if they are actually, actively attempting to recruit people to commit violence - that isn't free speech. That's conspiracy to commit violence. The answer for a free society, then, is "sometimes."
***Edit: It also depends on who is "allowing" said speech. "Exterminate all Jews in the world" shouldn't, of course, be "allowed" by admins in Christians Forums, for example, but it isn't in every wise something that should be *illegal* in a free state.***