This thread will provide you with all of your answers...and it is just over a year old and therefore quite accurate. There are at least 35 American zoos that average over one million visitors annually, and I've now been to 33 of them. The two that I'm missing are coincidentally located in the same city (Tampa):
1. So many areas had awful summer weather (floods, record highs for weeks, etc.) which kill attendance.
2. When the "Great Economic Melt Down" began, "staycations" and the like were popular and zoo attendance in 2008-9 was high. This past year there was a drop off
But it should be easy to gauge. Google attendance at a few of the zoos and compare to the list to get a sense of the trend