He's there to answer whatever they ask him. Like being a monopoly. And CNN seems to think they blew it:
Mark Zuckerberg
emerged unscathed from Tuesday's Senate committee hearing, and he did so in large part because most of the senators who asked him questions had no clue how Facebook worked, what the solutions to its problems are, or even what they were trying to achieve by calling its CEO to testify, other than getting some good soundbites in.
What the first day of the Zuckerberg hearings made clear is that many American lawmakers are illiterate when it comes to 21st century technology.
As a result, the issue that was supposed to be the focus of the hearing -- "social media privacy and the use and abuse of data," as Sen. Chuck Grassley put it -- was but one among many. And at the moment when the country needed a smart conversation about privacy, what it got was meandering questions and misfires.
http://money.cnn.com/2018/04/10/technology/senate-mark-zuckerberg-testimony/index.html