Within hours of the publication of a New York Post article on October 14th, Twitter users began receiving strange messages. If they tried to share the story—a dubious “exposé” of emails supposedly from the laptop of Hunter Biden, son of the Democratic presidential nominee—they were told that their tweet could not be sent, as the link had been identified as harmful. Many Facebook users were not seeing the story at all: the social network had demoted it in the news feed of its 2.7bn users while its fact-checkers reviewed it....