Facebook, Twitter, and YouTube have all been blocked in China.

Google is available in China, but the Chinese government ordered Google to censor search results. This, and an alleged attempt by the government to hack Google’s source code led to Google shutting down its Chinese servers. Google searches can still be performed in China, but they are routed to the company’s Hong Kong servers. Google’s revenues in China have gone down in China since the move. 

YouTube has been blocked in China since March of 2009. China also blocked Twitter, but Chinese activists still access it and use it to communicate to each other through foreign servers. Facebook has been blocked since July 2009, after ethnic riots in Xinjiang. Since then, Facebook clone websites have been popping up in China, making it much more difficult for Facebook to catch on in China if it is ever allowed back in. 

New laws in China could make Skype illegal too.