People have been amazed at the growth trends of Google+, and yes, they are remarkable. Just look at this graph created by Leon Håland, based upon data produced by Paul Allen which shows the time it took Google+ to reach 20 million users when compared to its closest competitors Facebook and Twitter:
Holy crap. Data like this invariably leads to people writing stories about how "Facebook is scared of Google+," and how "Google+ put Twitter on notice." But is it fair to draw these conclusions from this data? I don't think so.
Facebook and Twitter were the Pioneers, and as such their job was much, much harder by comparison. Facebook and Twitter for example had to overcome people's fear of these types of services, they had to build the social graph from the ground up, they had to prove that people talking about what they had for breakfast actually had value. G+ didn't have to do any of this.
Let's compare the growth rates of San Francisco versus that of Los Angeles. The chart below roughly shows the rate at which each region grew from roughly 100,000 to 1.2M people over different periods of time.
While not near as asymptopic as Google+, the chart shows that it took roughly half as long for Los Angeles to surpass one million residents than it took San Francisco. And why is that? Well, before the Gold Rush of the 1840's there were no railroads that could take people quickly and safely West. The people who built San Francisco did all the hard work, making it much easier for the people who followed them.
And the same is true for Google+. So who cares if Google+ is growing fast. Of course it is. The real test will be in sustained usage and in any evidence that might suggest that the introduction of another social network will "steal" usage from another service.