Few numbers...

"China, by official count, has 114 million migrant workers who have left rural areas, temporarily or for good, to work in cities, and that doesn't include tens of millions of family members who moved with them. Government experts predict the number will rise to 300 million by 2020, eventually to 500 million. Today, Shanghai alone has three million migrant workers; by comparison, the entire Irish migration to America from 1820 to 1930 is thought to have involved perhaps 4.5 million people."

From "In a Tidal Wave, China's Masses Pour From Farm to City", by JIM YARDLEY, September 2004