I think that the key difference between the human filter bubbles of old (in Germany we call that “Stammtisch“, after the table where the village pub's regulars meet to mutually confirm their shared beliefs) and the new filter bubble in social media is that the latter gives more of the appearance of professional journalism, with all the expectations we associate with that. And even if what we see in the social media bubble actually is professional, balanced journalism, it's still a much narrower selection compared to what we saw in the age of mainstream mass media.

When we watched TV news or skimmed a newspaper, we would also selectively focus on our favorite topics. But other topics at least momentarily grazed or attention while we mentally tuned out or skimmed for more interesting headlines. With highly personalized online media (and yes, the social media filter bubble is just one method to achieve that) we are losing that weak connection to out-of-focus topics. The worst part is that we do not even notice, because when our personalized digest contains a certain amount of content that would be on general news we feel our desire stay up to date already satiated.

If real news (as in what would have made it into mainstream mass media) would somehow disappear from our personalized feeds (leaving them with only the necessarily personalized stuff like niche hobbies, friends and very local content), we would go back to consuming more mainstream news channels again, making us less vulnerable to viral fakes and the like. Unfortunately, this is not anywhere close to a solution since there is no way Facebook could do this (and they would not want anyway).

> Even the KKK gets freedom of speech and assembly.

Yes, but their freedoms stop well before lynching people.