UPDATE----10 years later, having heard and read more about this topic----While what I wrote below may technically be true, I think there is indeed good reason to believe that pornography on the internet poses a real danger to Orthodox Jews as well.
A considerable number of current orthodox Jewish religious leaders, halachic authorities, and publications are hesitant or disinclined to permit internet use. I believe one of their principal reasons for objecting to internet use is the danger that the user will end up employing the internet for religiously inappropriate or prohibited purposes, in particular the accessing of pornography and other forms of sexually explicit material readily available on the internet.
In determining his attitude toward internet use, the halachic decisor must consider how much weight to assign this danger. (He must also evaluate and weigh the need for and/or benefits of internet use, but that is not the subject of this post.) In this regard, I have heard numbers bandied about regarding the widespread use of the internet for pornography and related purposes, presumably from surveys of American society; possibly from surveys of other industrialized countries as well. My input on this matter is merely the following: Studies regarding internet use for pornography etc. in society will not necessarily help our halachic decisor determine how much weight to assign this concern. Allow me to explain.
Consider a study designed to determine the likelihood that an American who enters into an A&P supermarket will buy and subsequently eat non-kosher food. My guess is such a study would find that a considerable percentage, probably a majority, will buy and eat non-kosher. Should our halachic decisor therefore conclude that no Jew should walk into an A&P, because of the great statistical danger that if he does, he will end up eating non-kosher? Obviously this argument is absurd. The flaw in it is that the study considers a population sample consisting mainly of people who have no reason not to eat non-kosher food, whereas an orthodox Jew does have such a reason. The study offers no data about people whose values include eating only kosher. Consequently, the study will not assist in predicting the likelihood that an orthodox Jew will buy and eat non-kosher food if he walks into an A&P. It's the wrong study to answer that question.
It is reasonable to assume that a considerably higher percentage of orthodox Jews than of the American, or Western, population at large consider not looking at pornography and other sexually explicit material to be a major value. Therefore, analogous to the A&P example, a study about internet use for pornography in any Western society will not assist - at least not much - in predicting the likelihood that an orthodox Jew will look at pornography if he uses the internet. Therefore, if such studies are quoted in this regard, their limited predictive powers regarding orthodox Jewish behaviour should be acknowledged.
(Obviously, this argument does not apply to a study specifically (a) about orthodox Jews, or (b) about a population sample from which one can reasonably extrapolate conclusions about orthodox Jews.)