I just learned of two ways of telling how many people are using Google+
1. Total number of Google+ Profiles :site:plus.google.com -inurl:about
2. Total number of Posts :site:plus.google.com -inurl:posts
If you haven't used the :site: command in Google you need to give it a try. Select the text and highlight, Right Click, and select “Search Google for” on these links
Right now I get 186 million profiles, but probably only half of that are daily users. Yesterday I blogged about that facebook had 10 times more users, but according to Khalil only 9 times more unique users.