This is an announcement from the pixiv team.

As of 28 Jan, 2012, after 1601 days of managing pixiv，we have finally reached 4 million users！ We would like to thank all users from the bottom of our heart for using our services.
The Usage Condition and registered user data are as follows :

・Usage Condition
Monthly page views Approx. 2.8 billion
Total submitted works Approx. 24.5 million works

・User Data Transition
2007
September 10 Began management
September 29 10,000 users（in 19 days）

2008
January 7 50,000 users（in 110 days）
March 18 100,000 users（in 190 days）
June 24 200,000 users (in 288 days）
September 4 300,000 users（in 360 days）
November 2 400,000 users（in 419 days）
December 30 500,000 users（in 477 days）

2009
February 8 600,000 users（in 517 days）
March 20 700,000 users（in 557 days）
April 28 800,000 users（in 596 days）
May 20 900,000 users（in 618 days）
June 26 1 million users（in 655 days）

2010
January 10 1.5 million users（in 838 days）
May 22 2 million users（in 985 days）
October 24 2.5 million users（in 1140 days）

2011
March 29 3 million users（in 1296 days）

2012
4 million users January 28（in 1602 days）

We hope you will continue enjoying pixiv in the future!