While that's true, for the amount of network traffic they're likely moving around, I wonder where they're placing their servers.
eg something like AWS with massive data transfer costs, vs something else like carefully placed dedicated/colocation servers at places which don't charge for bandwidth
If it's AWS, they've surely got a huge discount. No way they're paying 8+x normal big-fish CDN rates for transfer. At their scale, it would have easily been worth the effort to move to something cheaper than AWS long ago, or else to negotiate a far lower rate.
eg something like AWS with massive data transfer costs, vs something else like carefully placed dedicated/colocation servers at places which don't charge for bandwidth