The problem is that nobody wants to present their users with a gigantic list of contacts on first login and ask them to sort and organise them into groups. Instead, everybody just lumps all the contacts together so that the user is least inconvenienced at first, only to later experience problems. Google is in a pretty good position to tackle this, since a fair number of people who already have Google contacts probably also have their contacts sorted into groups for email, and that's a good representation of their real-life social network.