The CEOs of Facebook, Google and Twitter will face lawmakers once again on Wednesday — this time to defend the legal liability shield that underpins their business models. For Facebook’s Mark Zuckerberg and Google’s Sundar Pichai, it will be their second time testifying before Congress this year, following July’s antitrust hearing in the House. For Zuckerberg and Twitter’s Jack Dorsey, it will be the first of two sessions before Congress within the next month. The two agreed to testify before the Senate Judiciary Committee in November after Republican members voted to authorize subpoenas.