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").
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").