Facebook and other social media have become sources of news — and also of satire, which is often false, even though it reads like news. Actor Macauley Culkin is dead; Paul Krugman has declared bankruptcy; Sara Palin has joined Al Jazeera America. Those are all examples of the kind of fake news that appears on Facebook's News feed — where it often goes viral. Worried that its credibility is at stake, Facebook is asking its users to separate fact from fiction. Marcus Wohlsen is a former AP reporter, now a senior writer for Wired and author of Bio Punk: DIY Scientists Hack the Software of life.