So the idea yes to make an App something between Whatsapp and some dating site.
Functions :
-People can talk on a distance from 0 to 200m
-They can save their conversations
-They should be able to connect their profile with Facebook
-They can communicate with 3G or Wi-Fi
-Communication with their phone number
-GPS localisation so we are able to know the distances between people
-People should be able to add other people as friend on Facebook through the App
I hope this gives you an idea.