As most of you know internet in China is censored. The program also called as Great Firewall of China.
How does this censorship actually implemented in such a volatile and free environment of cyber world. Interestingly this is a very organized effort involving mass scale operation, cyber infrastructure and a daunting number of human resource. (read - http://www.ww24.org/2013/10/internet-censorship-in-china.html ) Censorship is a joint effort between public and private enterprise.
Social media platform like facebook, twitter are blocked to be accessed from Chinese mainland territory. You may wonder what else is blocked. This can be simply checked from the Great Firewall of China website.
Updated link (5-May-2018): https://www.comparitech.com/privacy-security-tools/blockedinchina/
You can enter the website which you want to see if is blocked in China and the query server tries to access it from 5 locations in mainland China including Beijing, Shenzen, Inner Mangolia, Heilongjiang Province & Yunnan Province and shows the result. Surprisingly many Google services are also blocked in China.