This essay from the Hoover Institute is worth a read for anyone discussing either online speech in general or the embarrassingly wrong pieces on Sec. 230 which have appeared in both Vox and the Washington Post in the last few days. Click here to read the full version in as accessible a format as possible without having to download the document yourself and tag it.
United States
The defamation (and negligence) claims against Twitter are blocked by 47 U.S.C. ยง 230.
This is worth a read by anyone who either wants social media to be classified somehow as a public utility, as well as by those who insist that their First Ammendment rights are being violated when social media platforms remove content they find objectionable. Worth reading also are the linked sources.