Because the Internet is full of drunk Australians and Americans discussing guns, China sensibly blocks most of the web from the sensitive eyes of it's Chinese citizens.
Sensible because who wants to see a bunch of English people discussing the Queen and tea non-stop but that also means that the Chinese people can't read this blog which is a real shame as i think they would enjoy it.
It is unlikely that China will relax its internet access laws anytime soon so they have created their own sites to replace Facebook, Twitter and Youtube which we can access.
In China they get on Renren, send Weibo's through Sina Weibo and access clips of cats doing cute things in Youku and as every fifth person is Chinese, that's a lot of people Weiboing and Renrening each other.
Alas Australians will always be drunk, Americans will always be obsessed with guns and the British will drink tea by the bucket so China will not experience this bloggers views on things but just in case there is an internet cafe somewhere in Beijing offering access to Western sites for a few renminbi, 你好中国 不要担心 您不丢失.