Facebook Inc. shares posted their steepest drop since 2015 as U.S. and European officials demanded answers to reports that a political advertising firm retained information on millions of the social network’s users without their consent. Politicians on both sides of the Atlantic are calling on Chief Executive Officer Mark Zuckerberg to appear before lawmakers to explain how U.K.-based Cambridge Analytica, the data-analysis firm that helped Donald Trump win the U.S. presidency, was able to harvest the personal information. Facebook has already testified about how its platform was used by Russian propagandists ahead of the 2016 election, but the company never put Zuckerberg himself in the spotlight with government leaders. The pressure may also foreshadow tougher regulation for the social network.