The above chart shows a historical view of the load factor for all U.S. airlines. Load factor is calculated as passenger-miles as a proportion of available seat-miles and shown as a percentage. As you can see U.S. airlines have been doing a good job of increasing their load factors in recent years. Even in the current recession (with declining passenger miles) U.S. airlines are managing load factors well. Load factor is a primary measure of operating efficiency in the airline industry.
Data source:
U.S. Bureau of Transportation Statistics T-100
=================================================================================