Facebook CEO Mark Zuckerberg faced calls on Monday from U.S. and European lawmakers to explain how a consultancy that worked on President Donald Trump's election campaign gained access to data on 50 million Facebook users. From a report: Facebook's shares fell more than 7 percent, wiping around $40 billion off its market value, set for their biggest drop since September 2012, as investors worried that new legislation could damage the company's lucrative advertising business. "The lid is being opened on the black box of Facebook's data practices, and the picture is not pretty," said Frank Pasquale, a University of Maryland law professor who has written about Silicon Valley's use of data. Lawmakers in the United States, Britain and Europe have called for investigations into media reports that political analytics firm Cambridge Analytica had harvested the private data on more than 50 million Facebook users to support Trump's 2016 presidential election campaign. Further reading: An undercover investigation by Channel 4 News reveals how Cambridge Analytica secretly campaigns in elections across the world. Bosses were filmed talking about using bribes, ex-spies, fake IDs and sex workers.