I'm going to toss in my personal anecdotal evidence in here: Where I used to work, of the 50+ employees, only a handful (~6) has personal subscription, and would have received such a survey. I filled mines out, I wouldn't know if any of the other ones who got them did or not.
Looking at those salaries, and I do think it's probably skewed higher, but again, as people have pointed out, it's all going to be very relative to both companies and location. I'm sure if you laid out a chart, you'd see a rising trend as you head towards west coast, and you would also see a significant drop if you look between Canada and US.