"In China, Facebook was blocked following the July 2009 Ürümqi riots[2] because Xinjiang independence activists were using Facebook as part of their communications network"
https://en.wikipedia.org/wiki/Censorship_of_Facebook

https://en.wikipedia.org/wiki/Intern...rship_in_China
So in essence, China are blocking things that might give their citizens "stupid ideas" :P

Here's a list of blocked stuff: https://en.wikipedia.org/wiki/Websites_blocked_in_mainland_China