What was the reason for the downfall of the Russian Empire in 1917?

For hundreds of years, an elite, awesomely wealthy Czarist regime ruled Russia, the rest of which largely consisted of peasants. That all came to an end during the February Revolution of 1917, which was precipitated by a number of economic, social, and political causes.

As Russia became engulfed in World War I, over fifteen million men joined the army, which left a shortage of workers for the factories and farms. This led to widespread shortages of food and materials. As goods became more and more scarce, prices went through the roof, and soon famine consumed Russian cities. Tempers in the lower class grew short, as factory workers were suffering 12- to 14-hour days with falling wages and unbearable health and safety provisions. Labor riots and strikes broke out everywhere. On the farms, things weren't much better. For centuries, a small class of noble landowners controlled a huge number of indentured servants, who were in essence tied to the land (serfs). In 1861, Czar Alexander II emancipated these peasants and gave them each a pittance of land to farm. The small amount of land they were each given to farm proved insufficient to feed and provide for a family's basic needs, and thus mass riots broke out in the countryside, too.

How did Nicholas II react to all the hardships his citizenry endured? Mostly, by ignoring them, and occasionally with violence as he'd send troops to squelch a riot or end a strike. His popularity sank, and in 1915, things went from bad to worse when Nicholas decided to personally oversee Russia's floundering efforts in World War I and left his dingbat of a wife in charge of the government. Eventually, the Russian people came together and suggested establishing a constitutional form of government. Nicholas, true to form, ignored the request, which led to the February Revolution of 1917 and the collapse of Czarist Russia. One year later, Nicholas II and his family were executed.