How the number of active users at Facebook has grown:
December 2004: 1 million.
December 2005: 5.5 million.
December 2006: 12 million.
April 2007: 20 million.
October 2007: 50 million.
August 2008: 100 million.
January 2009: 150 million.
February 2009: 175 million.
April 2009: 200 million.
July 2009: 250 million.
September 2009: 300 million.
December 2009: 350 million.
February 2010: 400 million.
July 2010: 500 million.
July 2011: 750 million.
September 2011: 800 million.
February 2012: 845 million.
Source: Facebook.