IMO the reason it failed is the same reason it was great. It didn't have Facebook's stickiness and way of getting you to add everybody because of the way circles worked. If it had been less privacy minded and just copied Facebook's lack of respect for user privacy and glued everyone together in one big pot, then it would have done better.
Well, not for me. Google Plus worked great for its users, but Google clearly wanted something else from it and kept messing with it. Changing or removing features, first requiring real names, then importing all Youtube users (that's when it really went down hill), and ultimately they decided to pull the plug despite millions of happy users. It was a fantastic success, just not as big as Facebook, nor apparently something Google knew what to do with.
