Google+: A ghost town?

Amir Efrati about The Mounting Minuses at Google+ for the Wall Street Journal:

Visitors using personal computers spent an average of about three minutes a month on Google+ between last September and January, versus six to seven hours on Facebook each month over the same period, according to comScore, which didn’t have data on mobile usage.

I wonder how they managed to sign up 90 million users while only a fraction of them are actually using it after all?