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.
"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 companies did not immediately respond to emails and phone calls seeking comment on the probe.
This is the latest in a series of regulatory actions against the country's top tech firms as China's cyber authorities adopt an increasingly hardened stance on censorship, doling out harsh punishments to firms that fail to comply.
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