He's not under oath, [from what I've read] the majority of the committee members have received campaign donations from FB, and some committee members own FB stock. In other words, the market knows that this is a charade.
Either it is campaign contribution or perhaps without understanding technology, most are afraid what Mark can learn about them running queries on their personal messengers.
Never ever seen them so nice, including questions if he wants a break now to which crowd laughed; they treated him like he is 8 years old. Incredible! No wonder he shook his head smiling for more questions!