Donald Trump understood at least one thing better than almost everybody watching the 2016 election: The breakdown of a shared public reality built upon widely accepted facts represented not a hazard, but an opportunity.

The institutions that once generated and reaffirmed that shared reality — including the church, the government, the news media, the universities and labor unions — are in various stages of turmoil or even collapse. Because Mr. Trump himself has little regard for facts, it was easy for him to capitalize on this situation. But even as Americans gobble up “fake news,” there is the sense that something crucial has been lost. A North Carolina man told The Times that while he regularly clicked on links to stories claiming that Hillary Clinton was indicted or that Mexico built a wall along its southern border, he missed the days when Walter Cronkite delivered the news to the nation.

He’s not alone; it was different then. Americans knew that whatever they were hearing on the news, their neighbors were hearing, too. Cable TV fractured that shared experience, and then social media made it easier for Americans to curl up in cozy, angry or self-righteous cocoons.

The rise of social media has been great in many ways. In a media environment with endless inputs and outlets, citizens can inform and entertain one another, organize more easily and hold their leaders accountable. But it also turns out that when everyone can customize his or her own information bubble, it’s easier for demagogues to deploy made-up facts to suit the story they want to tell.