My sources are people who went through the applications this year and got the jobs at the firms mentioned above. Interviews were very problem-intensive for the analyst positions. I don't know about PR, "front desk" positions, but I'm only talking about the analyst stuff.
Have you seen this:
http://economix.blogs.nytimes.com/2011/12/21/out-of-harvard-and-into-finance/?_r=0. Before the financial crisis, we were sending over 40% of our students to the finance sector (that's what I mean when I say "Wall Street" - the same definition that you understood in your first reply). Even after the crisis, the numbers stabilized at around 35%. More recent figures from Career Services puts it around 40% again. So yes, my university sends 40% of its students into Wall Street (finance).
I do not contend that networking is a must if you want an analyst, asset management, i-banking position. But ceteris paribus, quantitative majors are preferred. By quantitative, I mean engineering, physics, math, chemistry. Now, the reason you don't see all quantitative majors in those Wall Street jobs is that the supply is exhausted before the demand. Out of my major, probably around 20-30% want to go into finance, although that figure is presumably higher for math, engineering, etc. since the overall figure is, again, around 40%. Now, Princeton is not representative of the other Ivies, since other Ivies, yours included, sent far fewer kids to Wall Street. Most people who really do want to go to Wall Street major in international affairs, politics, or economics. Now there are several possible reasons for this. Nowadays math and science are hard. There are many reports out there lamenting how American students are falling behind their peers in math and the sciences and drop out of those fields. Simply put, many people do not enjoy being told they are wrong, which happens much more frequently in the sciences. Therefore, only people who truly love the sciences will major in them and of that subset, it is less likely that those people want to go to Wall Street.
I think your argument there strikes to the heart of this thread. People tend to go into fields they love not because of future expectations (which will surely disappoint) but because they enjoy the subject matter. Indeed, it takes thorough enjoyment of the material to suffer through two intensive years of science/math training. It is not for the faint of heart. That is why the OP's professor was probably offended. He thought the OP was majoring in biology only because he was pre-med, not because he loved the material. That is a subtle distinction and they are not mutually exclusive ends, but to the professor at that moment, it probably seemed that way.
So the point is this: not everybody who wants to go to Wall Street majors in STEM because those fields are simply not appealing to them for a variety of reasons. Instead, they devote their study to economics, international relations, etc. - fields that genuinely interest them. Now, my school has a quantitative "finance" track in economics that trains students in precisely the quantitative skills in high demand by finance firms but again, that is a small subset of the people who do end up going to Wall Street. Why do you think even intern salaries are so high? Firms have to attract top students away from engineering/science fields which are usually very well-paid - they do so by offering even more competitive salaries.