Surrounded by a stream of passengers on an MTR platform in Hong Kong on a regular weekday, a couple paused to pose for photos next to a station sign to mark their journey. “We took photos at most of the metro stations we visited, just to record our metro route,” said Arale, a 36-year-old tourist who was visiting the city with his wife, Qing, 37, for the first time. Recording such a visual diary is part of a popular trend on Chinese social media in which people travel around Hong Kong to take...