Instagram has 80 million users and 4 billion pictures

"The incredibly popular photo sharing service Instagram has hit 80M users, marking a 30M user increase since its acquisition by Facebook.
The announcement was made on the Instagram Blog, where the team announced the new numbers and also said that users have shared almost 4 billion photos.
Just after it was acquired, Facebook announced that Instagram had some 50M users. Shortly before it was acquired, it launched an Android app and said it had some 30M users, and marked The service surpassed 25M users in March of this year.
This is how Instagram has grown since October of 2010, according to Quora:
Oct 13 2010: 100,000
Oct 20 2010: 200,000
Oct 28 2010: 300,000
Dec 21 2010: 1 million
Feb 01 2011: 1.75 million
Feb 15 2011: 2 million
May 03 2011: 3.6 million
May 26 2011: 4.4 million
Oct 31 2011: 12 million
Dec 5 2011: 14 million
Jan 24 2012: 15 million
Mar 11 2012: 27 million
May 1 2012: 50 million"
Source:  The Next Web, 26th July 2012
Also - the slightly less informative blog post by Instagram