"Users are spreading violence, terror, false rumours, pornography and other hazards to national security, public safety, social order", the regulator said on its website.
The Cyberspace Administration said it was investigating Tencent Holdings' WeChat, Weibo and Baidu's forum site Tieba over suspected violations of the country's strict cyber-security laws.
China's president Xi Jinping has made cyber sovereignty one of the government's top priorities and reasserted the party's role in guiding and limiting online discussion.
But despite the tight surveillance and censorship, dissent still bubbles away and, ahead of a highly sensitive Communist Party Congress this autumn, the authorities are tightening those controls further.
The companies did not immediately respond to emails and phone calls seeking comment on the probe. But while Apple is trying to maintain good relations with China by complying with such requests, analysts and tech commentators believe that its troubles with the country 'have just started'.More news: 'The Walking Dead' Creator Robert Kirkman Is Leaving AMC for Amazon
More news: In Japan : Government approves US Marine flights of Ospreys after crash
More news: Liverpool owners say Philippe Coutinho is not for sale amid Barcelona interest
Shares of the Hong Kong-listed firm were in the red after the news, down nearly 5 percent.
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.
Social media sites in the West such as Twitter and Facebook have been banned by the censors in the country, which helped to in turn increase the popularity of WeChat the China based messaging app and Weibo the microblogging service.
Last month, Beijing scrubbed memorial photographs of democracy advocate Liu Xiaobo (劉曉波) from WeChat and Weibo, following the long-imprisoned Nobel Peace Prize-winning writer's death in the middle of the month.
Regulators over the past few months have taken unprecedented and severe moves to eliminate content and media on a number of different platforms. In May, it released regulations for online news sites and network portals that expanded curbs on content and required all services to be overseen by party-sanctioned editorial staff.