I'll never understand how US court system basically allow you to randomly force anyone unrelated to a case into it and suffer the legal cost. It make it so easy to weaponize any case to your advantage.
The legal cost aspect is unfortunate. However, as others have mentioned, the court probably doesn't require the assembly of new documents, but rather submission of existing ones. So while there is a cost, it's not devastating.
The interesting question is whether or not this specific subpoena has real merit, or if the court was played by Meta.
As it gets more and more weaponized the slow arm of the law will move to prevent it.