In China, Facebook was blocked following the July 2009 Urumqi riots because Xinjiang independence activists were using Facebook as part of their communications network. Some Chinese users also believed that Facebook would not succeed in China after Google China's problems. Renren (formerly Xiaonei) has many features similar to Facebook, and complies with PRC Government regulations regarding content filtering.

What's the best answer to this question: is it illegal to use facebook in china?
yes