Well-established? You mean like the studies of YouTube radicalisation, where researchers take content creators with more than one account and then arbitrarily classify one as 'far right' and one as conservative, as a means of 'proving' that watching YouTube leads the far right radicalisation? The scholarship deployed to produce such studies is so shockingly poor, even laymen can spot the flaws clearly.
What I will concede is that social media has continued and amplified the process of polarisation which was initially created by corporate media, and is amply argued by Matt Taibbi in his book Hate Inc.- but that's a market force, with providers tailoring their content to specific audiences, rather than trying to present facts-based information to a broader swathe of the public. Both Tucker Carlson's and Rachel Maddow's lawyers argued in court that no reasonable person could mistake their content for news.
How do we know that Trump and Brexit were symptoms of a bigger underlying problem? Because populism has happened in America before- no less than four times, always as a result of high rates of foreign-born citizenship and always emerging as a result of a major economic downturn. In the past, conventional politicians curtailed immigration, simply robbing the demagogic figures which inevitably emerged of their support. This time, that didn't happen- in the UK we got Brexit and in America you got Trump- by failing to respond to legitimate economic insecurity and the disappearance of 'a job for life', the political class handed the keys of the kingdom to cretins and facilitated economic disaster.
There were sensible approaches which could have been taken. Raising the rate of employer taxes on non-citizens to 30% for one. Making it legal to discriminate in favour of citizens over non-citizens (which probably would have only has a symbolic effect). Adding major tax subsidies and incentives for training native-born young people (and Dreamers) in specific higher value blue collar roles, so that they weren't so thoroughly outcompeted by off-the-shelf fully trained foreign workers. But nobody wanted to address the legitimate concerns of the blue collar class, dismissing their views as either racism or bigotry (which admittedly was present in both America and Britain, but only for very small percentages of the population at the extremes). To my shame, I was one of them. For years I had looked down my nose at Brits who were only interested in sun, sea and booze on holiday, who ate British food in foreign lands, when there were whole worlds of culture and food to explore. It was only in the aftermath of Brexit that I began to read first 'The Righteous Mind' and then more widely into the literature on ingroup.
Besides bias (or ingroup preference) isn't bigotry (although laws do generally need to be codified to prevent discrimination in multi-ethnic societies). The literature on ingroup preference shows that outgroup hostility isn't necessarily the natural consequence. It requires specific triggers. In the documentary on one chapter of Steven Pinker's work, The Violence Paradox, Harvard researchers were working on introducing equality in a culturally diverse Middle Eastern setting with view to trying to reduce outgroup hostility (although people are generally hardwired for fairness, not equality). The other main trigger is economic competition to the extent that it lowers living standards, which is certainly true for both the American blue collar class and the British working class. Unfortunately, the American and British political and economic landscapes have been supplying these triggers in abundance for some time.
It's far worse in America than it is in the UK- but then we tend to acquire your worst habits about five to ten years later.