Now, of course, being the newcomer Google+ benefits from already having Facebook and Twitter out there to spread the news about Google+ ... and to spread the links to Google+ material.
Google also has the massive directory of users of Google services... from Gmail to Google Docs and everything in between.
So on one level it's no surprise to see the phenomenal growth... still, it's quite impressive by any measure.
P.S. And of course I am on Google+...
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