More trend data from their annual reports:
There is a clear decline in the past five years. While some of this is probably due to institutions using the NSSE on a less than annual basis, I think there is a real decline here (the fact that schools may be using it less frequently is also an indicator of how useful they are finding it). My prediction is that this decline will continue as institutions shift to actual measures of learning, such as the CAAP and CLA.
[I did not include data from before 2003 because their reports were not clear about the number of institutional participants. The 2003-2005 numbers are estimates taken from the previous year’s annual report. These are US and Canadian institutions combined.]