Some of the world's top business leaders saw their net worth plummet in 2012, writes Simon Goodley. Facebook CEO Mark Zuckerberg lost about $5 billion as his company's shares went south, while Mexican mogul Carlos Slim lost $1.7 billion as a number of key investments were hit by Europe's economic malaise. "The poor lamb is now only worth north of [$70 billion]," Goodley writes.