They're not required to divulge documents. They're required to respond to the subpoena. An acceptable response is that you don't want to divulge because they're confidential or the request is too burdensome.

I believe the stance of the justice system is that Facebook's lawyers will behave ethically with the information because it's a professional obligation for lawyers.

You and I might see that as a laughable claim - unfortunately the justice system is run entirely by lawyers.