Google+ has an estimated 65m users, and is adding 625,000 users a day

"Google+ is adding new users at a very rapid pace. It may be the holidays, the TV commercials, the Android 4 signups, celebrity and brand appeal, or positive word of mouth, or a combination of all these factors, but there is no question that the number of new users signing up for Google+ each day has accelerated markedly in the past several weeks.
Each week my team from elance runs hundreds of queries on various surnames which we have been tracking since July. We revised our model based on the actual user announcements made by Google on July 13th and Oct 13th.
Here is what the tracking shows so far:
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 13th)
November 1 - 43 million
December 1 - 50 million
December 27 - 62 million
January 1 - 65.8 million (forecast)
February 1 - 85.2 million (forecast)
What is really remarkable is that nearly 1/4th of all Google+ users (24.01% to be precise) will have joined in December alone.
If this rate of new signups (625k daily) continues then Google+ will reach 100 million users on Feb. 25th and 200 million users on August 3. They will finish 2012 with 293 million users."

Source:  Paul Allen of Ancestry.com on his Google+ page, 27th December 2011