It was a concept that techies seemed to like, but restricted its viral growth by design. In reality, not many people care enough about their "social graph" to curate and restrict. It's sort of why Google+ really diminished its "Circles" concept - a large percentage of people made a few circles and just lumped everyone in those, rather than multiple heterogeneous circles.

Seems like good idea in theory, but what if you have 50 friends and you want to add a new one? The network will force you to delete one friend.