I think social media has amplified unreason more than it has amplified reason, which can only erode further our already diminished critical thinking skills. Part of the reason for this is the format most social media sites favor. Twitter, for example, demands short statements at the expense of lengthier more nuanced thoughts. As a result, communication is increasingly in the form of assertions rather than arguments. But social media aside, America has a long tradition of anti-intellectualism which tends to go well beyond what is generally seen in other developed nations.
You place some of the blame for this on a strong central government in the United States. I actually think the US has an incredibly ineffectual central government as central governments go. The feckless Trump administration is causing states and cities to take initiatives that previously they were more reticent to take. Their willingness to take up the Paris Climate Accord after the Trump administration walked away is a case in point. To be fair, increasingly bold efforts at the state and local government level began before Trump. His abandonment of any pretence of governance has only served to accelerate that trend considerably.
I also think it’s a bit too easy to blame the so-called “elites” for everything. For one thing, this assumes this ill-defined collection of persons is a monolithic group with a single agenda. No group on the planet can be accurately described in this way. The “elite” don’t get together regularly to create media talking points. Some of them are very concerned about climate change, others deny it. Some are quite philanthropic while others are miserly. Some are extremely well-educated but not all that rich while others are extremely rich but not all that knowledgeable about the world they inhabit, or even all that interested (i.e. Donald Trump again).
So, to make a long response shorter, I agree with you that a decline in critical thinking skills doesn’t explain everything that is currently going on in the United States, but it’s an essential part of it. Likewise, I agree it’s difficult to document this decline statistically, though we see the evidence for it everywhere. The US population has consistently been far more reticent to accept well established scientific theories, for example. It has also consistently demonstrated a great deal of hostility for policies proven to get better results at lower cost in other countries. The American tendency to label just about everything “socialism” demonstrates not only a lack of familiarity with socialism but a propensity on the part of Americans for quickly labelling ideas they don’t like so they don’t actually have to consider them on their merits. In many parts of the country state and local school districts have effectively abandoned science and critical thinking education, which is a fact readily reflected in test scores. Civics classes are likewise a thing of the past in virtually every state, and an education that includes some exposure to the arts is now widely seen as a luxury. Philosophy was never a major focus and is now a subject that must always wait for university. Even then it is optional.
Critical thinking may always be a rarity, but it’s the de facto policy of the US education system to make it rarer still. At some point, people have to be exposed to controversial ideas and actually wrestle with them seriously if they are to have any hope of developing strong thinking skills. Americans tend to get upset and defensive whenever faced with such challenges to a degree we just don’t see elsewhere. This isn’t just a problem among conservatives. We see it among the illiberal left as well, and it’s very troubling.