Will ask, can't promise an answer, but will post it as a child comment here (or edit this one if it is within one hour).

I think the collapse of public trust was very intentional, and the result of a much longer term effort than social media.

The most famous examples are likely the tobacco industry spreading misinformation through self-funded studies and experts, and the fossil fuel industry doing the same to seed doubt about climate change. But of course we can think of countless examples of entire industries and individual large corporations pushing out misleading bullshit, threatening or outright killing journalists and activists to cover up their catastrophic fuckups and their chronic conscious excretion of negative externalities.

This has all of course been going on since the dawn of time, but to focus on the last century in the US, we've seen all sorts of corporations and coalitions of rich and powerful people push misinformation into nearly every sector of our society - universities, science, journalism, politics, etc. in order to undermine confidence in shared facts, corrupt people's ability to discern whether or not something is fundamentally true, and sow confusion so that they can continue to operate in perpetuity in this chaotic maelstrom of doubt.

Lots of capture of government towards these ends as well, we can look at the concomitant constant cuts to education in order to weaken people's understanding of the world and ability to think critically. The revocation of the Fairness Doctrine was probably a step change, and Trump represents the sharpest recent escalation of all this.

From day one, he's done everything he can to shred any collective notion of shared objective truth. Anything he doesn't like is fake news, and the idea that the media is lying, scientists are lying, experts are lying, and institutions are lying, he has spread so fucking successfully through society, to the point where Americans no longer have anything like a shared sense of reality.

It seems like we're being reduced to tribes who are organized primarily around faith in various charismatic individuals.

I think this is fundamentally the worst thing he's done, because it lays the foundation for virtually every other conceivable and inconceivable abuse. If people can't even agree on what is happening, we're fucked. People and institutions in power can do anything they want to whoever they want, because the public has lost their ability to even recognize the danger posed to them collectively and thus mount any resistance based on a shared sense of reality.

Social media has definitely famously accelerated aspects of this like the fragmentation and the spread/magnification of fringe worldviews through echo chambers, but I think it's just one (and maybe this is controversial, but I'd be willing to be generous enough to think the 20something year old creators were too stupid to conceive of these long term consequences at first, but who knows, maybe not) element in a much longer and more intentional, malicious war against the many for the benefit of the few.