A pivotal paragraph from a skeptical Google+ review; check the link below for more analysis
There are three basic ways to share stuff online these days: You can do so publicly, for everyone to see; semi-publicly, for only your "friends" to see; and privately, for only designated people to see. Each of these modes is well-served by existing technologies: Twitter lets you share stuff with the public at large. Facebook lets you share stuff with your friends. And for private sharing, there's e-mail, texting, IM, Google Docs, the phone, and, from what I've been told, something known as "face-to-face conversation."