Photos of Zuckerberg's visit to Baidu Inc. were quickly posted online.
"Obviously I'm not going to deny what's in the pictures," Kaiser Kuo, Baidu's director of international communications, told The Associated Press by telephone. Zuckerberg had lunch with Baidu CEO Robin Li, Kuo said, adding that he didn't know what they talked about. He said the two had met before.
Not many in China are familiar with Facebook, but Zuckerberg is known for being Time magazine's 2010 Person of the Year, according to Kuo.
China censors Internet content it deems politically sensitive and blocks many websites, including Facebook, Twitter and YouTube. But curiosity has been high about Zuckerberg's visit to China and any possible business angles.
Zuckerberg has expressed a business interest in China, saying during a speaking engagement in October, "How can you connect the whole world if you leave out 1.6 billion people?"
Facebook watchers have also noted the travel-related questions he posted recently about China.
"Mark has had a long personal interest in China," Kuo said, pointing out Zuckerberg's efforts to learn Mandarin.
Zuckerberg is traveling with his girlfriend, Priscilla Chan, and no apparent entourage.
Aside from a visit Monday morning to a Tibetan temple in Beijing, the rest of his schedule in China is not known.