Meair I took this out since that thread was mainly dedicated to those who know about it, can share their ids for others to add.
For a discussion, a seperate thread is better.
I personally don't know much because I am not on facebook and haven't used the friends system, so others correct me if wrong.
Diggy's Adventure has in-game friends but how they end up there? It's through facebook. Only people who have facebook connected to their game or play on facebook can have friends. You just have to send befriend anybody on facebook and if he plays Diggy's Adventure too, then he will appear in your in-game friends list.
The add friends system here, on portal, has no real purpose. Just to show that we are friends, these friends do not appear in the game.