Many countries dislike social networking sites because they allow free speech. China are not able to control what their citizens say and do on these sites, so they block them. In fact China are rapidly moving towards the most restrictive internet policy in the world (apart from a few exceptions like North Korea and Iran).
At the moment they have a system based on a blacklist - where they block particular sites, there are indications that they will soon move to an internet based on a whitelist - that is you can only access websites that appear on this list.
This of course is much more restrictive, but is probably due to the problem that there are many ways of bypassing the traditional censorship model by using proxies and VPN tunnels.