At least in the USA…
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I cannot sue a telephone company for defamation of my character by someone’s speech about me in a telephone call. The service provided by a telephone company is a PLATFORM.
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I can sue a magazine company for defamation of my character by what someone wrote about me in the company’s magazine. A magazine company is a PUBLISHER.
Social-media site executives will say that users may post anything that they want as long as the executives insist that the sites are PLATFORMS and NOT PUBLISHERS.
These executives do this to comply with the “Section 230” law in the USA, which gives social-media sites immunity from defamation lawsuits as long as they act as PLATFORMS.
The truth, though, is that social-media sites for years have censored posts on a daily basis. That is, they control their content. This makes them de facto PUBLISHERS.