Also a very old idea of mine :) But in part that's what reddit does. Also a general-purpose geographic forum, where you can leave notes to people in a certain area ("I lost/found this item around here"/"beware of the crazy old lady leaving poison for dogs in this area"/"beware the child toy in the park catches on children hands; make sure they keep their hands above the seat at all times"). I also thought about tinder before smartphones had finger touch screens.

Which goes to show execution is everything.