Google Plus has an estimated 43m users

"If the algorithms are all working right, then Google+’s “unofficial statistician” Paul Allen believes that Google+ now has over 43 million users. Allen, whose name you might know as being the founder of Ancestry.com has a method whereby he estimates the total number of users on Google+ based upon uncommon surnames.
In the past, according to PlusHeadlines, Allen has been startlingly accurate:
July 4th – 1.7 million users
July 9th – 4.5 million users
July 12 – 10 million users
September 9th – 28.7 million users
September 22nd – 43.4 million users
These numbers tie up almost exactly with confirmed reports that we’ve seen in the past, as well as our own findings from Google employees."

Source:  The Next Web, 22nd September 2011