I think Medium is a good option. It has a wide readership and allows for different styles of short stories. You can reach a diverse audience there. Also, some people like posting on personal blogs or websites to have more control over the presentation and audience interaction.
For putting short stories online, Tumblr can be a choice. It has a community that appreciates creative writing. Another option could be Archive of Our Own (AO3), especially if your stories have a certain theme or genre that fits well with its user base.