My solution: Group people by zip code, give distance ranges based on the two users' zip codes.

Within same zip code or two zip codes that border each other? "0 feet to (farthest distance between any two points within zip code(s))"

Zip codes farther apart? "(closest distance between any two points within zip code) to (farthest distance between any two points within zip code)"

No need to limit queries, no need to fuzz anything. No way to triangulate unless one person happens to live at the corner of three or more zip codes, in which case you could just pin them to one zip code as long as they are within a certain range of that zip code.

In areas where zip codes are unusually large (Alaska?) or unusually small (NYC?), you can substitute in some other chunk metric.