Studies on media inflluence on public opinion

They say that human beings use just ten per cent of their brains. They say that non-cholesterol fat is better than saturated fat. They say that small scrawls in a stone have proven that there was life in Mars. They say that our children's exams qualifications are decreasing. They say that Jesus was a King David's direct descendent. They say that you can earn 15000€ on your spare time. They say that marihuana leads you to LSD and LSD guides you to suicide. They say that a job in an office is a symbol of power. They say that old people will have flue this winter. They say that homosexuality is a peculiarity environmentally learned. They say that there is a gen for homosexuality. They say that people under hypnosis can do everything. They say that people under hypnosis will not do anything that they would not do conscious. They say that Prozac relieves depression. They say that fix deposits are the best for a long term investment. They say that you do not know which kind of expenses decrease taxes.

Who exactly are they and why do they say so many things?.
And, the most important, why do we listen to them?