They were hiring in 6 cities, so that's an average of 30k applicants per location. That's a heck of a lot for a single community, even in a large city. How many businesses get tens of thousands of job applicants at a single location?

One of the locations is Arlington Virginia, the future location of HQ2, but they're only hiring 400 there now. In future they expect that will go up to 24k, which is about 10% of the population of the city!