What would happen if a CEO defied the gag order of a National Security Letter?

Google, Facebook, Microsoft, et al have apparently all received National Security Letters over recent years, compelling them to supply information about their users to the govt. and ALSO compelling them to remain entirely silent on the subject. So not only must they comply with the request, they can’t even talk about the request.

But what would happen if Mark Zuckerberg said, fuck it, the NSA and FBI are ruining our business, and I believe this law is a complete outrage and contrary to American ideals—and defied the order. Instead, Zuckerberg holds a press conference, detailing every single NSL his company has received, and encouraging the press to investigate them all, because he believes the law that enables them and requires him to be silent is unjust.

What happens to Zuckerberg? Fines? Immediate imprisonment? Noogies? What are the penalties for defying these letters and their gag orders?