Contrary to some of the other comments here, I've actually found great success on Reddit and HN.

Post about your project and position it as a story. Becoming a part of the community and creating an ongoing narrative there around your project is a great way to get people interested.

Here's a good example: https://www.reddit.com/r/rust/comments/1f8159z/the_allegatio...

This works better for free and/or open source products than paid, generally speaking. Lots of subs will allow F/OSS tools but will ban you for promotion of paid ones.

Thank you. Seeing real-world posts like this is incredibly helpful for me.