I have a strong suspicion that, if a government tried to fine a company in billions, that company would simply leave the country forever. I'm not saying that this is a good thing or a bad thing. It's just an intuition that I have.
I don't think there's much that would wholesale get Facebook to withdraw from a huge market like the US but if they did then competitors that did obey the new laws would take their place, as long as the entire business model wasn't completely defunct.
That said, I think the parent comment that suggests “20-50B” fines is dramatically overestimating what it would take to promote more depositor behaviour here. Even much smaller fines with the threat of larger ones would likely be sufficient.
The first move is usually to legally object to the fine if it's more than the cost of doing business, leading to years of back and forth. Look at the fine the EU imposed on Intel in 2009 which keeps getting contested and reexamined. Increasing the cost and friction for the ones trying to recoup the fine makes them more willing to negotiate a faster settlement (usually better deal for the company). It's a good bargaining chip for the company if they have very few assets under that jurisdiction or it's a market they can afford to lose. So the company pays some of the fine and then sees much stricter controls applied to them in particular, not via law but via the settlement.
Finally the company is represented by a CEO and/or board and those are the people ultimately responsible for disregarding a court decision. There could be attempts to hold them responsible but the US is famous for protecting any CEO from prison time (inside or outside of US borders). The US has a history of refusing to extradite CEOs convicted simply because they can purchase their way out of any trouble, it's only a matter of price. So this last step is mostly symbolic, the CEO is convicted because justice has to be served in the accusing jurisdiction.