To get custom computing gear built, you have to show your designs and plans and requirements to various potential vendors, so (and NDA or not) the vendors now know what you're doing.

If FB can cause these servers to be offered as standard configurations and produced in (much) higher volumes than when produced as custom solutions built for for FB (or Google), then the costs to FB drop, and FB can potentially also get the vendors to compete with each other.

It's a clever move.

(If you're just running big DCs, and not looking to move into the cloud hosting business.)