Why the surprise?
1. Zuckerberg couldn't give a rat's ass about the Dumb Fucks™ unless they start leaving FB in large numbers. Like most addicts since the first taste of poppy 48,000 years ago, they'll make endless excuses to themselves ("I am in control. I could stop anytime ...") and keep on fixing up.
2. The UK's DFs voted, 37% of them, to make the country less engaged, less influential, less effective, less powerful, less relevant and themselves and their country economically vastly weaker. The DFs' representatives, who might reasonably be trademarked as the Even Dumber Fucks™, in Westminster, are busily making a disastrous hash of this process.
3. The EDFs, especially those in the current ruling party, have spent three centuries demonstrating that they are fawning lickspittles of anyone or anything with cash, will bend over forwards and drop their trousers for big business at a moment's notice, and will do everything conceivable to avoid regulation which would benefit the schmuck citizens, even unto prating transparent tosh about "self regulation", "industry policing itself", "market ethics" and whatever else floats into their teensy brains.
4. Given the venality and stupidity of the EDFs and the nicely addicted, shallow gormlessness of the citizen DFs, why on earth would Zuckerberg waste a scintilla of his time posturing in front of such a circus of castrated halfwits? What would be the point? He gets paid more for picking his nose. Sitting in front of powerless commitee of self-important halfwits who have nucleated like a large human booger, will just make his nostrils itch.
5. The US, for all that it is currently "led" by the Dumbest Of All Fucks™ *¹, and under the sway of a laughably hypocritical, deceitful and deranged conservative party, has a history of sometimes severaly punishing bad corporate behaviour and making it stick*².
6. Therefore: Zuckerberg will appear before Congress, which has teeth and the wilingness to use them. He'll even fawn over the DOAF a bit, if it protects his cash cow. In front of the cameras he will mouth anodynes and platitudes to a bunch of fat, lazy twerps. Off-screen, Facebook's lobbyists will flaunt the contents of their wallets.
7. In short, he knows who matters now.
8. With luck he'll get one of his flunkies to send Theresa May a postcard saying "Facebook U".
*¹ The acronym DOAF did not, as Wikipedia suggests, originate with "Donald, OAF".
*² Banking excepted, I grant you: but when you understand how banking actually works, it is simply rotten beyond belief anyway. Even former US presidents have pointed out that if citizens knew how banking enslaved and impoverished them, they'd stage a revolution. Good job they are mostly DFs, eh?