I personally think Google's killer feature here isn't really a feature at all. It's the notifications on all Google properties.
As I'm using Gmail, search, documents, finance, whatever -- I get + notifications about friends, their updates, and photos. It's very distracting! But it also gets me to use the service more than I use Facebook -- and it's just started.
Facebook changes stuff all the time, and thus I would have to keep tweeking the settings. Instead, it all just goes into a circular file at Google which I empty from time to time.
And that's why Google+ circles don't aren't really a game changer - it doesn't significantly reduce the friction of managing notifications except for the small segment of people who are already interested in managing those notifications.
I had all e-mail notifications turned off from when I first logged into facebook. When they added the new groups I started getting e-mails again. I have to go into settings about once a month to turn off the e-mails in some different part of facebook.
You can add "+facebookcrap" (or anything else after the plus) to the end of your gmail address for things like that and the mail still hits your account, but it makes it really easy to filter out.
As I'm using Gmail, search, documents, finance, whatever -- I get + notifications about friends, their updates, and photos. It's very distracting! But it also gets me to use the service more than I use Facebook -- and it's just started.