Facebook has hired several upstream Linux kernel developers over the years, including several Btrfs developers. Their latest hire in the kernel department is Dave Jones.
Dave Jones was employed at Red Hat for 11 years before leaving to join Akamai. He ended up not liking the Akamai experience with lack of upstreaming kernel patches and other factors that left him rather unhappy. Thus after seven months he left the company and took an offer from Facebook. At Red Hat, Dave had been managing Fedora's kernel as well as over the years managing various components to the upstream Linux kernel. He had also been the developer of the Trinity fuzz tester, but as that article explains, he effectively ended the public development of that.
Dave hasn't shared what exactly he'll be working on with the Linux kernel at Facebook, but you can find his remarks about leaving Akamai via this blog post.