Google+’s Circle Logic
businessweek.com
businessweek.com
I think another non-trivial difference is that there no ads, at least not yet. Even if they add gmail-style ads, it'll be a lot better than FB. But more importantly, Google doesn't necessarily need to make money on Google+. They just need it to help make search better, so they can continue to make money on search ads, which is their real bread and butter.