Three months ago, Harvard student Aran Khanna was preparing to start a coveted internship at Facebook when he launched a browser application from his dorm room that angered the social media behemoth. His application, called Marauder’s Map — a clever name that Harry Potter fans will appreciate — was a Chrome extension that used data from Facebook Messenger to map where users were when they sent messages. The app also showed the locations, which were accurate to within three feet, in a group chat with people he barely knew. That meant complete strangers could hypothetically see that he had messaged...