Can I sue someone for defamation over a social media post in Ontario?

Yes. A social media post can absolutely form the basis of a defamation claim in Ontario, since it is published, meaning shared with at least one other person, the moment it is posted publicly or even shared privately with a limited group, and the same general elements apply as with any other defamatory statement: it must be defamatory in meaning, identify you, and be communicated to someone else.

Social media claims often raise their own practical challenges, though. Identifying an anonymous poster can require a separate legal process to compel a platform to disclose identifying information before a claim can even properly proceed against a named defendant. The rapid, wide reach of social media can also affect how damages are assessed, given how quickly and broadly a statement can spread compared to older, slower forms of communication. Deleting a post after the fact does not undo the fact that it was already published, though a prompt retraction can sometimes be relevant context for damages. The type of platform and audience does not change the underlying legal test, only some of the practical evidence involved in proving the claim.