There's a lot of ways to gather information about a topic. Surveys allow a lot of responses quickly and easily, but they have many limitations.
PC Gamer ran a survey for a couple of weeks, polling their audience about whether they pirate computer games, and why. It attempts to correlate this act by age, income, country of residence, and reason. It also asks about how this practice changed over time. They acknowledge that this system could easily be gamed, whether by multiple votes or deliberate misinformation, but noted that it's an interesting study none-the-less. They even highlight a few areas of concern, like non-zero income for people who claim to be under 10 years old (of which some are probably guessing their parents salaries, but still).
The survey is interesting, though, and you should check it out.
It's important to know a bit more about how surveys work, though. Simply put, people often report information that is much different from what would have been measured, especially in hypothetical or long-term situations. Someone who records what they ate during the day, through a survey that occurs multiple times per day, is likely to be fairly accurate.
However, asking someone if advertising works on them is hilariously bad. When I've seen surveys on this, they are overwhelmingly “no” or “it informs me of products or services I would have otherwise not been aware of”. Hate to break it to you, but that's crap. It works. It works on everyone. There is an industry that is worth hundreds of billions of dollars, in the US alone, that testifies to it working.
While you would typically expect a survey about piracy to be skewed in a way that makes the respondents self-identify better, even that is not necessarily the case. About a decade ago, Paulo Ceolho was pirating his own book, leading to an increase in sales. The same happened for a comic book artist, named Steve Lieber, whose sales peaked about ~20x higher than being reviewed on Boing Boing; this peak lasted longer, too.
These sorts of effects, as well as many others, will probably not come up in a survey. In the latter case, there is an emotional reaction to an author who treats you with respect, even though you pirate their work. You actually need to test for these effects with concrete experiments.
In short, read the data with a few grains of salt. This is not an effective acquisition method for what they are attempting to learn, but it's well done for what it is.