1 The Chinese government bans harmful sites
The Chinese government bans the websites it believes to be harmful. A big list of websites is banned inside china and Facebook is one of them.
2 Riots in Xinjiang
After a series of violent riots in Xinjiang that led to the death of 197 people, China banned Facebook.
3 It is used to co-ordinate protests
The Chinese government believes that Facebook and other social networks are used to Co-ordinate protests. This is why other social networks such as Twitter were banned as well.
4 Used to spread rumors
The Chinese government believes that Facebook is used to spread rumors that could threaten the safety of the people. See why Youtube is banned in some countries.
5 Facebook didn't give China access to users' info
One condition for lifting the ban was getting access to content and users' info. Such access was refused, as users didn't like it and so China didn't lift the ban.
6 To help national websites
Some people argue that the ban remained because China wanted to give local people the opportunity to create successful websites and so push the economy forward.
7 To control content
According to a Quora thread, the Chinese government blocked many sites with user generated content to have better control on what's being said on the internet inside china.