(Thanks to Vincent Docherty, Mike Glyer, and Cheryl Morgan for the data.)
I've also been given a more comprehensive set of numbers that list nominating figures 1971-2015, but they do not always align with the reports I have and at which I can document. The source of some of these figures are contemporaneous reports at the time, many of which were only on paper and have not been saved or documented, which is a pity.
The figures do show that, broadly speaking, nominating participation for 1971-2008 was generally static in a range of about 400-700 people per year. 2009 was the first year we see a significant up-tick in nominating participation from the previous few years.
What is unclear (and even now still is unclear) is whether the percentage of eligible members is actually increasing. WSFS has been steadily increasing the nominating franchise, bringing in first the previous year's members and then the following year's members, so that the eligible nominating electorate is he union of three years of Worldcon members as of January 31 each year, a group that could be more than 20,000 people at times, compared to the fewer than 5,000 previously eligible prior to the expansion of the franchise. It's actually possible that the percentage of eligible members participating has gone down even as the absolute number of nominations has gone up.