I'm aware, but usually the Cubs in the past few years have been sold out for every single game. However with the struggles of the last 2 years their attendance has really declined.
I was curious when I saw that and you're right - after their two NL Central title seasons in 2007 and 2008 when they averaged over 40k/fixture each season, they've declined continuously ever since:
2007 - 40,153
2008 - 40,743
2009 - 39,610
2010 - 37,814
2011 - 37,260 to date