That's the number of people moving to squatter communities in the world's cities, according to this piece by Jennifer Rowell, the urban adviser for Care International, in The Guardian. It's a shocking statistic (though I've done the math, and it's actually too low: the reality is that 2 people become squatters every second.) Rowell writes: "People in many slums are treated as though they have no rights to education, to clean water or to fight eviction. But those rights are theirs. And, as their numbers grow, so too does the need for urgent solutions." I'd second that, with this proviso: the people themselves are the root of the solution.
Rowell's piece was a response to a heartfelt piece on urbanization in China, which you can read here.
[thanks to Edesio for the link]