[FreeTrade]

Nice idea.

I'd encourage you to embrace full decentralization via blockchain technology. Why? Any successful tech that empowers and connects local communities becomes very politically interesting very quickly.

[carlnewton]

Thanks. Decentralisation is the aim. I don't think blockchain will be required though, because lat/long coordinates will act as a route directly between instances. I describe this here: https://carlnewton.github.io/posts/location-based-social-net...

[FreeTrade]

Not sure I understand your 'because' here. Blockchain is helpful in making data public and persistent, and not under control on any one individual.

You might also be interested in geohashing as a more user friendly way to group nearby posts, although lat/lon is easier for a cpu.