I went in December, and there were three cars in the parking lot. One of my friends told me that Pittsburgh teenagers learn to drive in that parking lot, and yes, it is usually that empty. The zoo was really cute and intimate, and the aquarium was OK too. One of the overall highlights for me was the gorillas; one was wearing a yellow jersey-type shirt. Very odd. The free-range peacocks are more mellow here than at other zoos, and quite beautiful.
The weather was a downer, which probably contributed to the lack of traffic on the day I went. In some parts of the zoo it was so empty it felt like I was the only one there. I guess that's the upside of going in winter!
Overall, the zoo is one of the better tourist attractions in Pittsburgh. It's been a couple of years since I've gone, and I wouldn't mind going back!