Normally, you wouldn't need Facebook to disclose any names because Facebook isn't anonymous 99% of the time. There are plenty of anonymous and pseudonymous forums that would be at risk and yes they too have to follow warrants should the court decide against them.
If Facebook wants to stay out of such cases, they should either leave the jurisdictions where such warrants are possible (so planet earth, probably) or they should enforce non-anonymous posts so plaintiffs can sue each other without involving a court warrant first.