I advertised an app I wrote on Reddit, not by posting, but by using Reddit ads. Say what you want about Reddit; their ads work very well. I was able to target relevant subreddits, people made posts about it(opening the door for me to interact with potential users), and I got a few hundred people to sign up through the Play store.

I didn't continue because I realized that there just wasn't a way to make the app profitable. I had assumed, based on my experiences from 10 years ago, that I could make money on ads, but ads today are pretty worthless unless you reach a critical mass of users.