Web2Asia's George Godula writes:
"As of today 8pm Chinese time Facebook seems not to be accessible from most parts of China Mainland anymore. On the China Telecom connection of our Shanghai office the service vanished at around 7:45pm. Friends in Hong Kong are reporting that they can still access the website."
A quick test on WebsitePulse confirms the blocking of Twitter and YouTube (which have been restricted for a while) and now Facebook too, at least in some parts of the country.