Now Playing: 10-04-13 Marketplace - TWTR

Twitter's business model relies on advertising, but it’s unclear how many of Twitter's user are real or robots. That's a question advertisers -- and investors -- are going to want to know. The health care exchanges are open. Now the real work begins, especially for the group of health care advisers known as navigators. We look at the week that was on Wall Street, with our Weekly Wrap. And finally, in addition to Tropical Storm Karen working its way toward the Gulf Coast, there’s an early winter storm brewing in the west. It comes as many FEMA and NOAA employees are furloughed (though at least a few have been called back).