The collapse of the 20th-century media pyramid on which Lippmann’s assumptions rested, and its rapid replacement by monopoly social media platforms, made it possible for the Obama White House to sell policy—and reconfigure social attitudes and prejudices—in new ways. ... the term “echo chambers” to describe the process by which the White House and its wider penumbra of think tanks and NGOs generated an entirely new class of experts who credentialed each other on social media in order to advance assertions that would formerly have been seen as marginal or not credible, thereby overwhelming the efforts of traditional subject-area gatekeepers...