Some indie manga creators post on social media platforms like Instagram or Tumblr. But keep in mind, the reach might vary. You could also consider creating your own website or blog if you want full control over the presentation and monetization.
You can try posting them on popular writing platforms like Wattpad or Archive of Our Own. These sites have a large user base and are known for hosting various types of stories, including horror.
You can try posting your manga on popular online platforms like Webtoon or Tapas. They have a large user base and provide a good opportunity for exposure.
Well, social media platforms like Instagram or Twitter can be good options too. Just make sure to use relevant hashtags to reach the right audience. You could also consider creating your own website or blog to showcase your manga and build a dedicated fan following.
I think posting on dedicated comic forums or communities such as DeviantArt can be a good start. They often have passionate comic lovers who are eager to discover new works. You might also consider pitching to established comic publishers' online platforms if you aim for a more professional reach.