I think it's interesting to note here that despite Facebook, Google, and Twitter's best intentions, new social networks grow faster today than ever before. Instagram, WhatsApp, and Snapchat exploded in a manner of weeks.
I should hope they got circles right as it was the most obvious thing in social networking ever. 30 seconds into my very first usage of facebook and I already realized it was missing what google calls "circles".
- and, unlike twitter they have a lot of space to move in (pay for api access anyone? Voluntarily subscriptions to feeds? )