This is a remarkable achievement for a brand new social network that is still in beta stage and just 9 months old since the launch in July 2011.
Here is +Vic Gundotra, Senior Vice President of Google has to say:
More than 170 million people have upgraded to Google+, enjoying new ways to share in Search, Gmail, YouTube and lots of other places. It’s still early days, and there’s plenty left to do, but we’re more excited than ever to build a seamless social experience, all across Google.We take this opportunity to congratulate Google and the Google+ team for this remarkable achievement. Great job guys!!