It depends on the time periods we are using. Compared to volume in the 50's, 60's, 70's 80's capacity is trending consistently upwards. Flights have become a fraction the cost of what they were back then and more people use it as transportation. Meanwhile rail and bus travel has been going down in the same time period. Thanks to LCC's (Low CLASS Carriers ha ha ha) like Southworst and Sleasy Jet flying has become accessible to the proletariat.

But looking at shorter periods I am sure we can see a decrease since 2008 when the economy tanked in earnest. I think we are not back up to 2006 numbers yet, at least that is something I read a few months ago.