1 million — End of 2004.
5.5 million — End of 2005.
12 million — End of 2006.
20 million — April 2007.
50 million — October 2007.
100 million — August 2008.
150 million — January 2009.
175 million — February 2009.
200 million — April 2009.
250 million — July 2009.
300 million — September 2009.
350 million — End of 2009.
400 million — February 2010.
500 million — July 2010.
608 million — End of 2010.
750 million — July 2011.
800 million — September 2011.
845 million — End of 2011.
901 million — March 2012.
955 million — June 2012.
1.01 billion — September 2012.
