By Megha Rajagopalan BEIJING (Reuters) – Chinas campaign against online rumors, which critics say is crushing free speech, has been highly successful in cleaning the Internet, a top official of the countrys internet regulator said on Thursday. China has the worlds most sophisticated online censorship system, known outside the country as the Great Firewall. It blocks many social media websites, such as Twitter, Facebook, YouTube and others, along with many sites hosted in Taiwan and those of rights groups and some foreign media agencies. High profile users of Sina Weibo, Chinas Twitter-like microblog, have been targeted, apparently for political speech.