It rapidly became clear that the burdens of curation of circles and figuring out which one to use made using GP too much effort. FB does have a mechanism like that but it's barely even known about, while the coarser "group" mechanism does seemingly 95% of what people would want out of circles.

One might consider it an example of "worse is better", though I'm not convinced.

(Wow, just checked and Google+ is still available. I'd thought it had already shut down like Orkut and Wave).