Two Different Approaches to Social Interaction
The difference here is bigger than it might appear. Google+ and Facebook represent two entirely different ways of thinking about social networking. Which social network you choose may depend on how you like to interact socially.
Facebook thinks that all social interactions are related, or should be related. You can’t separate one from another; there is benefit from linking (or networking) every interaction with another.
To that end, Facebook puts all your content and communications in one place. There’s very little organization or segregation; it’s one very large bucket of content. If you want to find something, you have to sort through everything else to get to it. It’s cluttered, it’s chaotic, it’s… well, it’s the way really life is, in many ways. Why should your social network be any more organized than the rest of your life?
Google+, on the other hand, serves up a social network that is as organized (or overly-organized) as you might wish your life actually was. It’s not cluttered or chaotic; it’s neat and clean and maybe even a little sterile. It’s organized in such a fashion that everything is in its place, which makes it easy to find things. It also makes it easy to deal with people, because you can determine who sees what. (You can also determine what you see and when, of course, which makes your grazing of updates more efficient.)
Looking at it this way, Facebook consolidates content; Google+ separates it. Facebook is egalitarian; Google+ segregates. Facebook gives you one big messy pile of stuff; Google+ sorts things into lots of neat little piles.
Or, to put it simply, Facebook does things the way life is; Google+ does things the way you’d like life to be.