It makes for a great headline, but the comparison of Google+’s early growth rate to Facebook’s early growth rate is folly.
When Facebook launched, it was an entirely different context. I’m not sure, but I’m thinking the idea of “social media” hadn’t even formed yet, let alone be considered a cultural phenomenon. Heck, you couldn’t even join if you didn’t attend a university. It was something for the “youngsters” and just for fun.
The only measure, in my humble opinion, is engagement. Are Google+ users spending more or less time in that network? Are they interacting more or less? Are their networks more or less vibrant? That’s what matters most, not how quickly or how slowly it is adopted compared to Facebook.