This is cool! I had been thinking about creating something similar but I couldn't solve this problem:

Given a radius of 5 miles and three users in a geographic straight line, each 4 miles from each other. User 1 and user 2 are in the same radius, user 2 and three are in the same radius but user 1 and 3 are not in the same radius. Therefor user 1 and 3 will only see "half" the conversation (which would obviously be confusing). Do you solve for this problem? if so, how?