It was tough going for Facebook chief executive Mark Zuckerberg on the second day of his testimony on Capitol Hill, but no less challenging for the congressmen who had to grasp the technical details and complex ramifications of the phenomenon unleashed by a 19-year-old in his Harvard University dormitory room in 2004.

On Wednesday, Mr Zuckerberg faced a series of questions not just on data security, but also on advertising policy and liberal political bias.