From the 2006 10-K:
During the year ending December 31, 2006, OnePass participants claimed 1.5 million awards. Frequent flyer awards accounted for an estimated 6.8% of our total RPMs. We believe displacement of revenue passengers is minimal given our ability to manage frequent flyer inventory and and the low ratio of OnePass award usage to revenue passenger miles.
(emphasis added)
For comparison, here's AA's 10-K blurb on AAdvantage:
The number of free travel awards used for travel on American and American Eagle was 2.6 million in both 2006 and 2005 representing approximately 7.5 percent and 7.2 percent of passengers boarded. The Company believes displacement of revenue passengers is minimal given the Company’s load factors, its ability to manage frequent flyer seat inventory, and the relatively low ratio of free award usage to total passengers boarded.
(emphasis added)