Facebook has spent a lot of engineering resources on the kernel. Can't speak to their network stack, but if you want to see what they've contributed, they use emails under the @fb.com domain:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

From a quick glance through I see a lot of work on eBPF stuff, memory, I/O subsystem, cgroups, and btrfs.