image thumb27 China bans Facebook, Twitter: Why? Maybe to suppress reports on riots, the riots themselves.. or just for the heck of it

I am not surprised, a riot followed by a ban on social networks and other online services. The same has happened once again in China as it banned Facebook and Twitter after riots in the western region of Xinjiang. China is already making the moves to automate the bans on websites by integrating the Green Dam with all new PCs in the region. I wonder how exactly does this help in blocking news about rots in a country from breaking forth to the international audience?