This week, Twitter announced that it's removing locked accounts from its platform. That's in an effort to clean up its bot problem. Bots are fake Twitter accounts that spread propaganda and intimidate people. A New York Times story in January suggested that up to 15 percent of Twitter's user base could be bots. Twitter says it was more like 5 percent. But now, as the company is trying to clean up the bot problem, some investors are worried that Twitter may have fewer human users than they thought. Kevin Roose follows social media for the New York Times. He spoke with Marketplace Tech host Molly Wood about how bots became a business problem for Twitter.