Facebook has a job posting for a ‘Linux Kernel Software Engineer’ ( emphasis mine ):
Facebook is seeking a Linux Kernel Software Engineer to join our Kernel team, with a primary focus on the networking subsystem. Our goal over the next few years is for the Linux kernel network stack to rival or exceed that of FreeBSD.
Why the comparison with FreeBSD? I suspect it comes from their $19 billion purchase of WhatsApp. They were claiming 2 million established TCP connections on a single FreeBSD server back in 2012:
Today we have an update for those keeping score at home: we are now able to easily push our systems to over 2 million tcp connections!
…
Best part is that we are able to do it with plenty of CPU and memory to spare and do it sustainably
The post describes a few details of their FreeBSD ( and Erlang ) setup.
I’m sure that Facebook would like to consolidate servers to a single operating system. It sounds like they can’t do that until Linux can handle a similar load ( with a similar performance profile ).