Google+ could have been successful to a degree, in terms of features it was an improvement over Facebook in several ways. The problem was the invite only launch.

The invite period worked for Gmail because it was still interoperable with other email services, and made getting a Gmail address seem exclusive and desirable. Making a walled garden social network invite only, however, just lead to it being empty. Most who did sign up looked around for a few minutes then went back to Facebook.