The growth of Google+ accounts looks like this:
|July 13||10 million|
|August 1||20.5 million|
|September 1||24,7 million|
|October 1||38 million (Larry Page announced "more than 40m users" on Oct 13)|
|November 1||43 million|
|December 1||50 million|
|December 27||62 million|
|January 1||65.8 million (forecast)|
|February 1||85.2 million (forecast)|
December has been especially busy for the people at Google. Almost one in four of all Google+ users (24.01%) have joined in December.
See Paul Allen's entry at Google+.