As the U.S. heads into one of the most perilous phases of American democracy since the Civil War, social media companies are scrambling to shore up their patchwork defenses for a moment they appear to have believed would never come. Most major platforms pulled the emergency break last week, deplatforming the president of the United States and enforcing suddenly robust rules against conspiracies, violent threats and undercurrents of armed insurrection, all of which proliferated on those services for years.