Last time a "Show HN" was posted with a similar nearby chat app, I was in Disney World and there was not much engagement ( https://news.ycombinator.com/item?id=16976287).

I'm in Disneyland now, and I've tried it out now, but not much engagement. I will check it throughout the day, but I mirror the suggestions made, either increase the 1 km radius (the park here is longer than 1 km) or include the closest 10 users.