The key data points were 400 million total users and 100 million monthly active users.
Pause for a minute. Twitter only recently hit 500 million registered users and most analysts put twitters monthly actives between 150 and 200 million. Does Google+ feel like it has even a fraction of Twitters adoption?
Twitter is wide spread, has viral growth and is a important part of our culture. News frequently breaks first there. Governments are overthrown as a result of organization that takes place on the platform. Massive companies have been born and killed as a result of Twitter. It is not forced when you include your twitter name on a business card or in a email signature. You put it there without after thought.
Yet, Google+ has none of that. The only mentions you see of G+ are fleeting celebrity endorsements and one-off events heavily coordinated by Google employees (Obama/Dalai Lama/etc). It is when you think of these facts that the true disparity between Google+ and its competitors feels real. It is forced and does not feel natural.