Оказалось это уже не так, FB в этом году начал разделять сеть для связи между DC и для связи DC с интернетом.
https://code.facebook.com/posts/1782709872057497/building-express-backbone-facebook-s-new-long-haul-network/
As new data centers were being built, we realized the need to split the cross-data center vs Internet-facing traffic into different networks and optimize them individually. In a less than a year, we built the first version of our new cross-data center backbone network, called the Express Backbone (EBB), and we've been growing it ever since.
https://code.facebook.com/posts/1782709872057497/building-express-backbone-facebook-s-new-long-haul-network/