Typically you want to bring traffic off Medium to your website or social, not the other way around, but I know that's not what you asked. What I do is take a screenshot of my article and use that in a social graphic with a bit of copy explaining the main points of my article or benefit of reading it and directing people to the article. You could also try recording a short teaser video clip of you explaining the same thing (Instagram story, Facebook, LinkedIn, etc.) and include a link to Medium. Finally, I know this isn't technically social, but you can promote your writing in your email newsletter.