Using ESPN.com’s attendance data I calculated total attendance as a fraction of capacity for each year going back to 2001 (as far back as ESPN has the data). In brief, I made sure it was weighted properly by essentially finding the total number of seats available at all games in all ballparks, then taking the total actual MLB attendance as a fraction thereof.
So far in 2012, 72.9% of all seats have been sold, the highest number in the entire available range back to 2001. Leaders are the Red Sox and Phillies, both of whom apparently are overselling capacity (though this is known to be a sham in Boston’s case, as local ticket agencies are required to buy up any unsold tickets for each game and Fenway itself rarely close to full on game day.)
Here is the fraction of total capacity sold each year:
2012 72.9 2011 70.0 2010 68.3 2009 68.2 2008 72.1 2007 72.4 2006 69.4 2005 69.4 2004 67.3 2003 60.4 2002 60.2 2001 64.3
Attendance reached a high (in both absolute numbers and fraction) in 2008, then started to decline as the US recession deepened. Since then, the economy has improved slightly, but capacity has also shrunk with, for example, the new (smaller) Yankee Stadium opening.
As average game attendance every season prior to 2001 was lower than the average in 2012, I think it’s a reasonable guess that fraction of capacity was also lower in all those earlier seasons. The only seasons with a higher average attendance that 2012 were 2007 and 2008, and as we can see, the fraction of capacity is higher in 2012. Of course, as the year drags on and more and more teams fall out of contention (or trade their stars as will happen in the next couple of days), attendance is likely to fall in many places and 2012 may fall out of first place.