The similar battle in newspaper and television included heavy moderation and legislation to limit this. E.g., do you think the Wall Street Journal was regularly publishing letters to the editor that focused on the arguments for abolishing private property?
The only legal question unique to social media is the 230 provision that protects a platform from legal action from third parties due to hosting user-generated content. That is still an open question. Social media companies always have the option to apply stringent moderation and declare themselves a publisher. They could even do this selectively. E.g., a newspaper with an online presence still receives 230 protection for user generated comments on an article when there is a clear denotation differentiating what is published by the site and what is user generated.