"For several years, Andrew Marantz, a New Yorker staff writer, has been embedded in two worlds. The first is the world of social-media entrepreneurs, who, acting out of naïvete and reckless ambition, upended all traditional means of receiving and transmitting information. The second is the world of the people he calls 'the gate crashers' -- the conspiracists, white supremacists, and nihilist trolls who have become experts at using social media to...