Facebook already has circles! Facebook groups is a distinct feature from facebook lists. Facebook lists are the same feature as Google circles. And the only reason you didn't know this already is that you did not care enough to look, so circles cannot be all that much of a killer feature. To utilize "Facebook Circles" first you must put categorize some friends into a list. https://www.facebook.com/friends/edit/ Click…
Facebook has always been about sharing every post with every person. You're right, the only difference is that g+ puts the UI in your face. You're forced to put people in circles, and it's stupidly simple. Facebook hides the feature, and in a community filled with highly technical early adopters, someone needs to write multiple step instructions to explain it.