If you compare the numbers of common stock issued from Part II, Item 5, under the paragraph Recent Sales of Unregistered Securities you'll see that as of October 31, 2015, they had issued a total of 4,409,716,191 shares of common stock. As of January 21, 2016, we had outstanding 2,944,402,694 shares of common stock.
That is a retirement of 1,465,313,497 shares of common stock.
This verifies the FORM 8-A released on Feb 10, 2016 which stated "As of the date of this Form 8-A, there are 2,944,402,694 shares of our common stock issued and outstanding.

I don't believe there have been any other statements by Bill, tweeted or otherwise, since the Form 8-A about further share retirement. So, it's 2.9 billion issued and outstanding. However, nothing would prevent him from reducing the share structure further, as he has indicated that he would like to do.