[r00fus]

I'll say one thing about Circles... Google doesn't really have any social users yet.

Facebook, to implement Circles as it exists in Google now, will have to retrain their users. Not an easy feat.

The concept of a quick pivot to add new features or markets is silly. If your users expect you to do it the old way, you'll face a long uphill battle.

[brudgers]

Circles are nice, but not a game changer. People are on Facebook because of features. They are on Facebook because of people.