Juan Campos has been working to save at-risk teens from gun violence for 16 years. As a street outreach worker in Oakland, California, he has seen the pull and power of gangs. And he offers teens support when they’ve emerged from the juvenile justice system, advocates for them in school, and, if needed, helps them find housing, mental health services, and treatment for substance abuse. But, he said, he’s never confronted a force as formidable as social media, where small boasts and disputes online can escalate into deadly violence in schoolyards and on street corners. Teens post photos or vide...