While I do love the visual transition that Google is making with its apps, including Google+, I think that Facebook's UI is near perfect. It has come a long way with negative feedback after every update, but the fact is that it works. Facebook has an iconic look and feel that people have come to expect, while Google is building theirs out out from scratch, pretty late in the game.
I came across an article on TechCrunch, written by MySpace Co-Founder, Tom Anderson, regarding Facebook's strategy, the new Skype partnership, and so forth. Read: Why Mark Zuckerberg’s First Public Response To Google+ Is The Right One. He points out may positives about what Facebook has going for itself.
The new "Circles" approach of organizing friends is something Google+ users will have to learn/get used to. We are generally good with adapting new methods, but there has to be an obvious "return" value. Why should we put effort into your new product when Facebook already works well?