Each message will be tightly coupled to a specific geographic location, fwiw. And messages will be ephemeral, they'll be deleted completely from the server after a few hours (or maybe a week).
So I'm thinking that there'll be a backend API that the mobile apps will use to send/receive messages.
My question is: what kind of technology would you use for such an application? Where would you store those messages?
Then I'd use PostgreSQL with PostGIS. Fastest way to get up and running and it will scale decently well. Well enough for a side project.
If you get traction and scale takes off, switch to something more exotic. Until then, a single PostgreSQL instance will work wonders.