MIAMI—The disappearance and death of Gabby Petito and the police hunt for her boyfriend have generated a whirlwind online, with a multitude of armchair detectives and others sharing tips, possible sightings, and theories by way of TikTok, Instagram, and YouTube. Whether the frenzy of attention and internet sleuthing has helped the investigation is not clear, but it has illuminated the intersection between social media and the public’s fascination with true-crime stories. Months before her disappearance drew more than a half-billion views on TikTok, Petito, 22, and 23-year-old boyfriend Brian Laundrie set out on a cross-country road trip over the summer in a van. They documented their adventure on video and invited social media users to follow along on the journey, sharing scenes of a seemingly happy couple cartwheeling on a beach, hiking on mountain trails, and camping in the Utah desert. But they quarreled along the way, and Laundrie returned …