How to detect invisible users in Gmail?

Are all Gmail account holders counted as Google+ users?

• I know that new Gmail users have to set up a Google profile in order to use mail, but what about Google users who have never to their knowledge used Google+ ?  Does having a Gmail account mean that you are a Google+ user in Google's eyes?

• Answer:

  No. Unless you use Google+ features within Gmail. According to Google's last official statistics on Google+ usage[1], they split "Google+ users" into three categories: Users who visit the Google+ stream at http://plus.google.com in their browser or Google+ app Those who use Google+ features across other Google products (starting Hangouts from Gmail, leaving reviews on Play Store, adding people to your circles from Search, etc) Those who have Google+ accounts So no, just having a Gmail account doesn't count you as a Google+ user (though if you have a Google+ account you will be counted among those who have a Google+ account). In order to be counted as a Google+ user, you'll need to either interact with the Google+ stream or use Google+ features within Gmail (managing your circles, starting/joining Hangouts, sharing items from the Gmail share button, etc). [1] http://googleblog.blogspot.com/2012/12/google-communities-and-photos.html, December 2012