Zuckerberg, of course, didn’t say that it had been. He said that free speech had helped the movement to succeed. That free speech was also deployed against the movement, sometimes to dangerous effect, does not refute the point. King and Ifill, like Zuckerberg, are implicitly asking us to consider a counterfactual scenario in which U.S. law and custom in the 1950s and 1960s had been less protective of free speech.
In that situation, would a benevolent and wise central authority have kept Hoover from spreading lies about King? It seems much more likely that Hoover and his allies would have been the central authority censoring their foes.
The risk that power will be misused is also why Warren and Biden are wrong to ask Facebook to adjudicate politicians’ claims. The truth or falsity of those claims is very often disputed. Consider the track record of fact-checkers. The Post’s recently slammed Senator Bernie Sanders for exaggerating the number of bankruptcies that are caused by medical bills. (I also think Sanders is wrong.) Would our political debate really be improved, though, if Facebook refused to broadcast Sanders’ argument or slapped a “false” rating on it? I doubt it: A Facebook that took such action would be likely to squelch too much true and valuable speech.