Facebook’s shares fell over 7%, wiping around $40 billion off its market value, amid investors’ fears that the US government may formulate new rules and hurt the company’s advertising business. The share plunge on Monday was the worst since September 2012. US lawmakers are calling for action against political data firm Cambridge Analytica tied to President Donald Trump’s presidential campaign. It is said to have illegally harvested data from millions of Facebook profiles. Cambridge Analytica has strongly denied the charge. An investigation into Cambridge Analytica’s role in the campaign could further hurt Facebook’s image. It already faces charges that Russians used its tools to sway American voters with “fake news” posts during 2016 elections. Cambridge Analytica will give a digital forensics firm hired by Facebook full access to its servers and systems to conduct an audit.