Last time they talked about it (that I know of -- when Fedora was contemplating using btrfs and asked Chris Mason et al for their opinion), FB were running data

Last time they talked about it (that I know of -- when Fedora was contemplating using btrfs and asked Chris Mason et al for their opinion), FB were running databases on xfs and were looking for ways to place them on raw disks for maximum performance. So not the entire stack.