Facebook CEO Mark Zuckerberg wowed students by speaking Mandarin on Wednesday at Tsinghua University in Beijing — a pivotal market that remains closed to the social network. Zuckerberg is touring Asia and speaking about his pet cause — greater Internet access. At the discussion at Tsinghua, where Zuckerberg serves on the advisory board for the School of Economics and Management, the 30-year-old tech mogul said that Facebook is already helping Chinese companies reach consumers outside the country.