I've considered this problem, even built a product to address it. A difficulty is that if its anonymous users won't know who there friends are and if you add a pin exchange system you are creating excessive difficulty to your friending system.

If you figure out a way to get around not knowing who other people are on the network while still being able to find your friends then you will be one step closer, but you will also have to figure out a system that is simple enough to be comparable to the easy of adding friends on Facebook.

After solving those problems, good luck getting over the network affect of Facebook's already established network.

Personally I think you need to look forward instead of backward, Facebook and Google products already exist and have an established user base, the next big thing on the internet isn't going to be a remake of what already exists, it will be something NEW.