I'm a very big fan of a Millian approach to balancing free speech with any other competing interests. I think the harm principle is a very useful instrument to frame the discussion because of its lowest common denominator quality.
As David van Mill (to my knowledge not related) states in his SEP article on freedom of speech, "If we accept Mill's argument we need to ask “what types of speech, if any, cause harm?” Once we can answer this question, we have found the appropriate limits to free expression". Van Mill goes on, of course, to the Millian corn dealers example, which poses the greatest headache to anyone trying to use the Millian approach in the XXI century. There are several instances where it is accepted that free speech may cause harm and criminal law offers exemples of this but still the discussion lingers on regarding the admission that free speech can cause harm.
As many know the corn dealer example sets the Millian boundary to what can and cannot be accepted as free speech. The problem being how to read it in the age of social media. The latest announcement by WhatsApp that it will restrict message forwarding to five times because of killings in India based on rumors spread through the app offers a tantalizing opportunity to test Mill's example.
John Stuart Mill famously said that it is acceptable to say in print that corn dealers starve the poor but not so in front of a corn dealer's house when an angry mob is standing right at the his door. The difficult question is: would John Stuart Mill consider WhatsApp users an angry mob and cyberspace the door of the corn dealer's house? Given that Mill's criteria seem to be the capacity for certain speech to produce harm linked to a present or imminent situation where such capacity may exercise social networks which have been used to everything from planning terrorist attacks to call for demonstrations on civil rights the problem centers on the credibility of the information concerning its possible audience. Mill speaks of an angry mob, angry here being the operative word. Today we witness great radicalization of thought in social networks where the invisible algorithm helps the confirmation bias. So oftentimes we do have angry mobs all over social networks and any speech that calls for violence may in fact create a plausible link to iminent or real violence. Problem: following this line of thought exponentially increases the number of cases where free speech may in fact violate the harm principle. WhatsApp self-regulation seems to show just that.
The problem does not seem to be in this exponencial increase but the two usual ways to deal with harm caused by free speech. Either there is self-regulation or State regulation (hetero-regulation). In this case there is a regulation of a different kind, somewhere in between both. It is the social media platform - WhatsApp - that is regulating the free speech of its users, not themselves and not the State. Mill would not care for self-regulation of free speech and this equals simply exercising free speech in other terms but it is more challenging to know what he would say of regulation by the newspaper. I'm inclined to say that he would consider it a violation of free speech unless the harm principle could be shown to be attacked by such free speech. And so the question remains: can social media cyberspace be the door to some corn dealer's house? Evidence says so and this seems enough to WhatsApp and in my view it should to be enough for a legal system. But is it a question of evidence? Mill does not tell us if his example if based on prior knowledge of similar situations. It is more plausible to believe that he simply believes that people under certain conditions and with contextual opportunity may turn to violence due to speech. It is up for legal systems to decide which conditions are those and what defines such contextual opportunity. This may be done through general precedent but also through controlled scientific findings relating to groups behavior. So in the end we are always talking about evidence. Social media just gave us access to more houses and more angry mobs.