Two days after the school shooting in Parkland, Fla., that left 17 people dead, David Hogg feared public interest in the massacre was waning. The 17-year-old student at the school trained a camera on fellow students at a park, began broadcasting live on Twitter’s Periscope app and asked them to describe life after the carnage. The segment drew more than 33,000 viewers, some chiming in with comments such as, “Keep speaking out. Be the change.”

“I started live-streaming so people could see, to reignite the interest,” said...