should be unnecessary since the DNS zone above it, facebook.com is already CNAME'd. Most resolvers will take a CNAME as "any further requests go to here", which to my experience usually includes NS servers.
(This is also why you don't CNAME your root domain, CNAME conflicts with any other record type)
(This is also why you don't CNAME your root domain, CNAME conflicts with any other record type)