At the CTBA Budget Blog, I wrote about that:

Historically, Chicago’s suburban collar counties have been the migration powerhouse of the state, hoovering up people from across the country and the world, while relatively few of those suburbanites ended up leaving. Cook County, meanwhile, saw huge net out-migration, as city dwellers moved to the suburbs. And the rest of the state hovered right around the break-even point.

The 2008 recession totally scrambled that old dynamic:

• The suburban counties, which since the 1990s had been gaining 10 or more new migrants for every 1,000 residents, suddenly fell into the red, losing more movers than they gained by 2010.
• Cook County, meanwhile, which just before the recession had seen a net loss of 13 migrants per 1,000 residents, improved dramatically. While it never hit positive territory, by 2011, it was actually seeing less net outmigration than the collar counties — a complete reversal of the usual pattern.
• The rest of Illinois, meanwhile, remained roughly the same as it had been through the worst of the recession.

There’s also a bunch of other stuff, so you should go read about it. (If you haven’t been paying attention, Illinois media and maybe some other people have periodic freakouts about the fact that more people are leaving Illinois than coming here, which then gets used as a political cudgel by pretty much everyone, and it’s a boring and frustrating conversation, but it turns out that what’s actually going on is pretty interesting.)