I have the same problem, my project is extremely niche and I hoped it would spread by word of mouth. I don't really use social media, so I don't self-promote it. The big boost came when I posted my project on a specific subreddit and it got a large number of upvotes. However the wave of users died off quickly and a year later I get like 10 users per day and maybe once per month it gets mentioned somewhere on the Internet.