Because the implementation is everything, and because Facebook added them after the fact, while Google+ is built with them from the ground up, as a central feat

Because the implementation is everything, and because Facebook added them after the fact, while Google+ is built with them from the ground up, as a central feature of the product. Facebook realized people will want to be separated in groups, too, that's why they launched both Lists and Groups, but they did it too late, and almost nobody used them. This is why maybe 5% or less people use Lists on Facebook, and everyone uses Circles on Google+, even if they are just using the default ones in the beginning, but they are using them, and learning to create others later on. It's really easy to do and intuitive.