Here is a graph constructed using FAA’s OPSNET data. It shows the total number of operations (takeoffs and landings) per year, from 1990 through 2013. The trend is clearly and strongly downward.
Combined FAA & Contract Tower Ops, 1990-2013
The Military component has been steady (see the red line, at bottom).
The Operations Network (OPSNET) is the official source of NAS air traffic operations and delay data. The data collected through OPSNET is used to analyze the performance of the FAA’s air traffic control facilities.
The data sent daily to OPSNET can be viewed on the FAA Operations & Performance Data Web site.
The Commercial component has been in decline (see the three blue lines). Note that there was a surge in smaller commuter planes (Air Taxi) peaking around 2004-05, with the explosion of feeders doing contract flying for the airlines. Since those peak years, though, the smaller planes are in decline, and commercial flights are being taken over by larger planes (Air Carriers).
The General Aviation component has seen a sharp and steady decline. Likely, an in depth analysis would reveal that high-end business/corporate flying and helicopter activity are increasing, while all other GA activities are simply dying. The reasons for the decline in recreational GA? Likely, due to high fuel costs and lower per capita discretionary income, and due to reduced interest (pilots are finding other, non-aviation activities to pursue). Also, it may be due to lack of aggressive promotion by FAA and others; i.e., whereas FAA and NASA did a lot to artificially promote GA in the 1990’s, we are now at the bottom of that promotional cycle.