Tens of millions of people all around the world do it, myself included. There must be something in it. [- anonymous blogger]
The one very sure property of our brain is that it does not work reliably and harbors plenty of habits of bad thinking. one of the very popular brand of bad thinking is the bandwagon fallacy or otherwise named ad-populum argument. In short it tells that a proposition is true because any number of people beliefs it to be true. Usually any number of people means the majority of the population. On the surface of it the argument sounds very plausible and persuasive. Many people lead their thinking and decision-making in their lives without realization that they fall into that type of fallacy. It is probably due to our innate propensity for listening to authority figures and the group of people provides enormous authority pressure to guide our thinking. However if we agree that the truth of the proposition should be independent on the source which expressed proposition or even formulated it, the authority might not be perceived as valid in that context. After removal of authority of the group from the argument it becomes void unlocking support therefore it becomes invalid.
Usually the best meets it to expose the fallacy is to show an example of argument of that type with up sort consequences of obvious to anyone.
We can for example point that Nazis ideology had massive support in the German society during Hitler's time, but this obviously doesn't render it true it in a factually or correct in a moral dimension.
The other example is the quite popular view in the past that the earth is flat. In fact only handful of people on the whole Earth suspected that in fact the planet might have in fact spherical shape.
We can generally say that any idea or ideology that has only large support in the society cannot at all be a measure of its validity. there must exist some other evidence in support of that idea to decide if it's true. The bandwagon fallacy arguments are very weak in their nature and can only be persuasive in case of unaware persons. It is a favorite tool of populist politicians, salesmen, priests or other persons want to test its power of persuasion to manipulate the crowds. However in more advanced debates such argument is used very rarely and usually indicates that the person using it is lacking best argumentation skills.