In theory, each user could see a different set of posts. The posts are sorted by proximity to you. So, there are no locality borders (actually there may be some due to implementation details, but it's no the intention).
I always thought something like this would be good for answering questions like "hey there's lots of cars here, what are they all here for?". But even for that, I would have thought time was a more accurate indicator of relevance than (approximate) location.