Does this really solve the problem though? What happens if someone 3 miles away is talking to someone who's 1 mile away from themselves, but 4 miles away from you? Then you'd see the transparent text of them talking to some person you can't see messages from, yet also see transparent text of them responding to people you can see messages from.

I personally don't see how you could completely avoid this without some kind of fixed regions, unless you required people to do some kind of an "@user Hey..." for every message so that you would know if you share the same sphere with the destination user. This makes it more tedious and less fun, though, plus it takes away the whole idea of multicasting.

This sounds like a solid and scalable approach, most others that come to mind relate to being able to join a conversation based on seeing part of it and then being added, but if the app isn't doing identity-based messaging (e.g. if A replies to B does it become more like tweeting at each other?) then there's no real way to do so.

Another nice aspect of keeping it strictly distance-based is that it makes it simpler to scale - all persons within a geographic region are able to see only other people within that region, so within limits you can run separate backend instances as required.

And of course there are all sorts of potential growth directions by allowing communities (e.g. "Students at XYZU and people within 1 mile" or "Students at XYZU who are within 1 mile").