Google+ is growing at the rate of 1.6 million users per day and has reached 236 million by one estimate.
For a late starter, Google seems to have hit a sweet spot to grow so fast. “The circles” is a hugely successful concept but the instantaneity of the system is wonderful. Integration with other services seems to be the icing on the cake. I am not sure how much more volume I can take on messages read… I seem to be my own bottleneck.