SHANGHAI (Reuters) - Some Chinese Internet users have this week been able to access blocked websites such as YouTube, Facebook and Twitter, relishing the newfound freedom although the reason for the breach in China's Great Firewall of censorship was a mystery. China blocks most foreign social networking sites (SNS) out of fear that unfettered access would lead to instability. Chinese SNS firms have filled the void by offering similar products that censor topics the government may find sensitive. ...