i think creating a story around your start-up and then posting this on related subreddits could be helpful for your start-up to go viral. r/Entrepreneur - r/sideproject these are really good ones!
By the way, friendly founders shared a lot of stories about how they tried Reddit and ... failed. Does anyone have a strong experience or cool use cases to share?)
It depends on the type of startup, and I personally use the platforms not only for promotion but also for comparison/dialogue with other users. I currently use: X, IndieHackers, Product Hunt, Reddit, AppSumo