Juanita Broaddrick had almost given up by the time she called her grandson from her house in Van Buren, Arkansas, on January 6, 2016. She had been trying, since seeing a video in late November, to send a single tweet, with no success. Her grandson, Ridge, then 13, diagnosed the problem immediately. “Nana, are there red numbers at the bottom?” Broaddrick remembers him asking. When Broaddrick said yes, he explained this meant that the message was over the maximum 140 characters then allowed by Twitter. Broaddrick didn’t tell Ridge what she was trying to tweet when he gave her the...