There is zero legal risk unless they decide to subpoena him, which they did not. The reason to show up is that these people have the power to make laws and mak

There is zero legal risk unless they decide to subpoena him, which they did not. The reason to show up is that these people have the power to make laws and make Facebook's collective life in the UK difficult, and so it is in Zuckerberg's interest not to piss them off. However, there may have been a calculation that him showing up wasn't going to make much difference to the opinions of MPs, and might provide video clips that were bad publicity.