How do you get the first critical mass of users for a social app? (i will not promote)

I still do not fully understand how to reach a critical mass of users. I built an app for finding friends and creating or joining small local events. I made a post on Product Hunt, and after that it was picked up by ukrainian media. Because of that, we managed to get around 1,200 users so far, with most of the audience coming from Ukraine. But now there is a problem. After that initial spike, new registrations have slowed down a lot. Right now, only the web version is live, while the mobile app is being pol