When national legislatures call people to testify, that's not a neutral information-gathering exercise. The MPs are going into that panel with a very clear idea

When national legislatures call people to testify, that's not a neutral information-gathering exercise. The MPs are going into that panel with a very clear idea of what the witness is going to say, and how they can spin it towards their preferred talking points. Zuckerberg isn't a UK citizen and didn't authorize any misconduct specific to the UK; why should he personally participate in Parliament's ritual shaming of Facebook?