His wisdom and personal belief (including legal and lawyer suggestions) tells him that coming over to the invitation would not do him (nor his company) any good.
And, given with all the information he has supposedly with people, he may already have some knowledge of the personality of the MPs who are going to grill him that he may think otherwise that appearing before them would tantamount into a gong show.
Practically speaking, he just snubbed an influential, cross-party group of MPs, and the House of Commons where those MPs and their colleagues sit has the closest thing to absolute power that exists within our legal system.
These are people who could pass emergency legislation imposing a windfall tax on Facebook amounting to 100% of everything it does or owns in UK territory and authorising HMRC to immediately seize assets accordingly. They could pass laws requiring all UK-based ISPs to filter Facebook traffic any way they want. They could pass laws imposing an extremely privacy-focussed data protection regime, even more so than the GDPR that is already due to come into effect in a couple of months.
Now, obviously such extreme reactions are unlikely in response to this snub, even given the history here. But there is a whole scale of possible responses to Facebook's current situation, and large fines and/or some of the people involved actually going to prison are not out of the question. Tempting fate may not be the smartest business move Facebook could make.