Lodging and Campgrounds refer to concessioner operations. Tent camping is camping in a tent attached to a vehicle or erected in a Park Service campground. Backcountry refers to camping at undeveloped walk-in sites not accessable by road. Misc. camping is camping in group camping areas, on boats, in undeveloped overflow areas, or in other areas not otherwise described. Non-recreation overnights are stays associated with non-recreation visits, including things like nights on board commercial fishing vessels offshore but within Park Service boundaries, or researchers on non-Park Service business.
The data comes from the Park Service's Public Use Statistics Office. The site has documentation as well.
Most types of overnight stays have been declining since the early to mid-1990s, or for over 10 years now. That certainly contrasts with trends in some other types of outdoor experiences - such as cruise ship visits to Alaska (Cruising to Alaska, Ben Muse, October 7, 2007). The Park Service declines are absolute, these are not nights per capita. Since they include foreign visitors as well as U.S. citizens, I can't express these in relation to U.S. population in a meaningful way. My guess though is that they do represent a per capita decline.
Are these declines due to income induced behavioral shifts, more and lower priced substitutes (video games), changes in fashion, changes in demographics (the aging baby boomers aren't so excited by tent camping any more, they want to watch their nature in comfort out of a cruise ship window), changes in reporting procedures, changes in patterns of park use?
Based on personal experience, I'd plump for the demographic explanation. My wife and I spent time in state and Federal campgrounds ten years ago with our kids, but now they're grown and we're older, I don't expect to be doing more of that. We haven't taken a cruise yet, but we're beginning to think it might be nice.
Edits Oct 25.