There is an interesting post at New Geography. It explores the just releases Demographia World Urban Areas and goes over the various changes in a variety of urban areas. By its count, 48% of the world population live within the 850 cities identified as having over 1/2 of a million people. Since a lot of people live in cities of less than 1/2 a million, the overall urban population is even larger still. But at a global scale it is hard to keep up with all the College Station, Texases out there.
The report did have one particular item of interest to us. The continuing depopulation of the rural areas. Note that this not just a relative (percentage) loss, but a reduction in absolute numbers. A reduction that is very close in numbers to the entire population of the United States.
World Urban Areas Population and Density: A 2012 Update
Wendell Cox, New Geography, 3 May 2012
The Continuing Exodus from Rural Areas: Around the world, people continue to seek the promise of better economic outcomes in urban areas. United Nations forecasts indicate that another 2.5 billion people will be added to urban areas by 2050, while rural areas (which contain all population not urban) will be reduced in population by 300 million. The world's urban population is expected to rise from today's nearly 53 percent to 67 percent. More than 90 percent of the urban growth is expected to be in less developed nations.