Google+ leverages the fact that you already have your “real” friends listed and possibly even organized in your address book. Google can leverage all of that behavioral information into helping you easily manage your relationships. Because face it–who you share with, how often, and with what other people you do that sharing provides valuable insight into the nature of the relationships.
I don’t expect Facebook to stay still for long. Look for them to roll out improved friend management tools in the near future. But regardless, they will always lack the behavioral intelligence to help me truly manage my friends, unless I am a devoted Facebook user.