Before www.SocialStatistics.com, there was no easy way to tell.  However, thanks to the hacking work of Boris Veldhuijzen van Zanten the mystery becomes an amazingly fun and interactive reality.  (You might have heard of Boris, he is also the founder of Twitter Counter, the premier statistics provider for Twitter.)  

Boris' new venture...

"tracks and generates detailed statistics for 39,803 users and pulls in hundreds of the most popular posts, every second of the day, from Google+. Add yourself (see sidebar) so we can start tracking your personal Google+ statistics too. You will be able to see how many people added you to their circles, how you grow over time and where you rank in the list of popular Google+ users"

To play devil's advocate for a bit, while the idea and concept are good, user adoption is always the critical component.  But, with 1 million page views and 300k unique visitors in the first week I believe this app is here to stay.  Especially being as G+ is not even out of beta!

Where's The Cash?  

The widget makes money by allowing people to become "featured" users of the platform.  Think of it as broadcasting your profile out to gain an increasingly large number of followers.