If it is exactly 1 km you could triangulate to find someone's location. A program to do that would do a bisection search along a line (faking coordinates for the location of the device) to find the point on the line where you transition to seeing the person's messages. Then, repeat along other two lines in different directions. Each transition point gives a circle of 1 km radius on which the person is present, and th…
Source: I've done this before and it is very accurate and creepy.
> This could be mitigated by rounding off coordinates to be coarse enough that it doesn't give their precise location before calculating the distance.
Reporting a slightly different location to users or varying the search radius per user search could help.