This is the Cyberspace Administration of China's first formal investigation of the platforms under the sweeping law, which tightens the government's grip on what passes through the country's computer networks.
The notice suggests the investigations were prompted by "reports made by web users" (language not uncommon in the enforcement of online controls), and that the three platforms "host content including violence, fabrication and rumor, obscenity and pornography, and other information shared by users that harms national security, public safety and social order".
In response, Baidu reportedly expressed regret and said that it would be willing to actively cooperate with the government to settle the issue.
China's regulations for suppressing the spread of censored contents will further assist officials to gain complete dominance over communication; hence, the researchers and new visitors to China are likely to face troublesome consequences without proper internet connection.
Weibo and Tencent did not make a comment when one was requested by email.
Shares of Tencent, a $380 billion company, plunged almost 5% in Hong Kong after the announcement Friday.
The move, which was unveiled Friday by the Cyberspace Administration of China, sent shares in some of the country's biggest tech companies sharply lower as investors fretted about the potential for harsh penalties against firms such as Baidu Inc.
Weibo is China's version of Twitter, WeChat is an instant messaging mobile application while Baidu Tieba is a famous discussion forum online.
Just last month, all three were asked to carry out immediate "cleaning and rectification" at a meeting with authorities who cited examples of illicit content, including rumors about party officials and misrepresenting Chinese military history.
Prior to the meeting, Weibo was ordered to partially close its video site over violations, wiping out a combined US$1.3 billion (S$1.8 billion) worth of stock between Weibo and parent company Sina Corp.