We'll see. I agree that Facebook Groups is much less powerful than Circles and much less fundamental to the core Facebook experience than Circles is with G+.

On the other hand, Facebook Groups takes less effort to use - each person does not need to sort their friends, a small number of curators can organize the group. More importantly, it is clearer to the user just what is going on. I have been trying to talk to as many non-techie non-early-adopters as possible about Circles, and many get it and love it, but several don't really understand it.

In particular, the asymmetry of circles is overlooked by a lot of people, even by some casual users who think they understand how Circles works. On Facebook, there is one authoritative list of who is in the group, and posts to the group stay in that group. On Google+, your circles are not my circles. I may have you in a "close friends" circle, you may have me in a big "co-workers" circle, or you might have all your friends in one circle. When I am browsing my "close friends" feed and see a post by you, I may guess wrong about the social context that you are sharing that message in, and make an inappropriate comment, not realizing who else received the message. (The way to determine on G+ who received a post is by noting and clicking on the gray "Public" or "Limited" in the upper right of the post) Maybe I am underestimating people, or maybe comments are just much less big a deal than posts, but I think this lack of clear social context for individual messages within Circles is going to lead to some serious privacy/sharing accidents.

If I had to make a prediction, I'd think that the ease-of-use and clarity of FB Groups will trump the power and flexibility of Circles. I could be wrong, so it'll be interesting to see what ordinary users prefer.