If there is a Subpoena or Search Warrant for FB messages, FB has no choice to hand them over. However, FB should have fought like hell in the courts to not hand those messages over.
They had a choice - they can tell their lawyers to fight the subpoena/warrant in court. Possibly a long, expensive, and futile exercise, but it would signal whose side they're on. And to the surprise of (hopefully) nobody, they're not on our side at all.
Isn’t it usually: “your archivist is required to appear physically at this address: xxxx, with a physical copy of the following items: yyyy. As a courtesy to you, you can email them to Special Agent zzzz if they are in the proper format.”
So showing up (at thousands of courthouses a year) is the default. If you’re helpful (to the prosecution), we will let you do things by email.