Reddit.
1. I made an app for the colorblind in 2015 and got my first 100 (and more) users from the r/colorblind subreddit.
2. I made a breathing app in 2017 and got my first users from r/breathing, r/breathwork, and r/meditation subreddits.
3. I recently made a productivity app for the mac and got my first users from r/macapps subreddit.
Reddit is incredibly powerful if you are building something niche and are already a part of the community. Also, the results are compounding because some of my posts get good SEO traffic so I still get a handful of users from Reddit every day.
replies(5):