Zackery’s phone filled in the gaps for Ms. Nazario, exposing her to a vast ecosystem of subway surfing content on social media that she believes ensnared her son. She had thought that social media was mostly a place where teenagers kept in touch with their friends. She scored a preliminary victory in June when the judge allowed the case to proceed to discovery, over the objections of lawyers for the social media defendants. (Ms. Nazario also brought claims against the Metropolitan Transportation Authority, which were dismissed.) “Leaders and transportation authorities have grappled with the challenges of subway surfing for decades,” the Meta spokesman said.