The daily forex trading volume evolution, in billions of USD :

2014 - 5500
2013 - 5300
2012 - 4700
2010 - 4000
2009 - 4000
2008 - 4500
2007 - 3300
2004 - 2100
2001 - 1500
1998 - 1700
1995 - 1300
1992 - 1000
1988 - 700

Source : Forbes, Reuters.