_spf.facebook.com has no CNAME records.
Question and response
Question
dig _spf.facebook.com. NS
Answer
The response has no answer records.
Authority
facebook.com. 3508 SOA a.ns.facebook.com. dns.facebook.com. 4207849484 14400 1800 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0