The Google+ circles concept is powerful and easy to use. It represents the defining, foundational difference between Google´s and Facebook´s vision for social networking. If this new model takes off with users, then Facebook will find itself in the uncomfortable position of having to replicate these features within its own platform. Unfortunately for Facebook, moving to this new paradigm will not be possible overnight. We are talking about a major architectural overhaul. In the meantime, Google will have a chance to attract significant numbers of users and influence.
There's a massive assumption here that G+ is blindsiding Facebook. Obviously there is no limit to human stupidity, however I'd personally be surprised if adding a circles-like feature to Facebook is an architectural nightmare, or that they haven't thought about it.
The idea that there are meetings at FB where people are saying "F___, we're screwed" is implausible. This has been discussed for years, and I'd bet actual money that FB engineers and/or product managers periodically propose adding this feature and FB decides that the time is not yet ripe. If and when G+ demonstrates that people really, really want this functionality, I expect Facebook will roll it out smoothly.