We knew there was a reason Mark Zuckerberg inhaled all that polluted air during his jog in China's Tiananmen Square in March. Facebook (FB) has been quietly trying to regain access to the Chinese market.

The New York Times has learned from three current and former employees that the company is developing software to prevent posts from appearing in people’s news feeds in China. According to the report, "Facebook does not intend to suppress the posts itself. Instead, it would offer the software to enable a third party — in this case, most likely a partner Chinese company — to monitor popular stories and topics that bubble up as users share them across the social network. Facebook’s partner would then have full control to decide whether those posts should show up in users’ feeds."

CEO Mark Zuckerberg, who earlier this year met with Chinese officials, has allegedly supported and defended the idea, but several employees who were working on the project have quit after expressing misgivings. “It’s better for Facebook to be a part of enabling conversation, even if it’s not yet the full conversation,” Zuckerberg said, according to employees speaking with the Times. (See also, Mark Zuckerberg Outlines Steps to Fight Fake News)

The news of Facebook working on censorship tools to appease the Chinese government comes six years after the website was banned in the country. During the 2009 race riots in the Xinjiang province, it was suspected that independence activists were using Facebook to communicate.(See also, Why Facebook is Banned in China)

Facebook has worked with administrations in other countries to censor content before. Its latest transparency report shows that the most content restrictions were applied in France, India, Turkey, Germany and Israel. But there's a key difference between this practice and what Facebook may be planning for China. Its official website says, "When governments believe that something on the Internet violates their laws, they may contact companies like Facebook and ask us to restrict access to that content. When we receive such a request, it is scrutinized to determine if the specified content does indeed violate local laws. If we determine that it does, then we make it unavailable in the relevant country or territory." In China, the task of choosing what content will be made unavailable will be performed by a partner firm most likely based in China.

In the past, Facebook has touted itself as a company that defends the principles of freedom of speech, but it looks like this is an exception it must make if it wants to be pragmatic about entering the Chinese market. China has the world's biggest internet user base. According to a McKinsey report from 2012, more than 300 million people in the country use social media, which is roughly equivalent to the combined population of France, Germany, Italy, Spain, and the United Kingdom, and Chinese users spend more than 40 percent of their time online on social media.Facebook would have to compete for marketshare with China's very own internet companies like Weibo and Tencent.