Marie Antoinette did not advise her servants to eat cake instead of bread.
The famous story associated with the French queen, wanders from school textbooks to the intellectual journals, and vice versa. According to this story, when Marie Antoinette was told that her servatns were starving, she replied: "If they do not have bread, let them eat cake."
True version of this conversaton is hidden somewhere in the bins of history. According to some information, this phrase was invented by the French philosopher Jean-Jacques Rousseau in 1766, when the queen was only 11 years old. The other story says that the sacramental phrase was uttered by another French queen, Maria Theresa, the first wife of Louis XIV.
In any way Marie was not the one who used this phrase.