Apex domains cannot have CNAME records.
Question and response
Question
dig fbsv.net. NS
Answer
fbsv.net. 5 NS b.ns.facebook.com.
fbsv.net. 5 NS a.ns.facebook.com.
fbsv.net. 5 NS d.ns.facebook.com.
fbsv.net. 5 NS c.ns.facebook.com.
Authority
The response has no authority records.
Additional
a.ns.facebook.com. 5 A 129.134.30.12
b.ns.facebook.com. 5 A 129.134.31.12
c.ns.facebook.com. 5 A 185.89.218.12
d.ns.facebook.com. 5 A 185.89.219.12
a.ns.facebook.com. 5 AAAA 2a03:2880:f0fc:c:face:b00c:0:35
b.ns.facebook.com. 5 AAAA 2a03:2880:f0fd:c:face:b00c:0:35
c.ns.facebook.com. 5 AAAA 2a03:2880:f1fc:c:face:b00c:0:35
d.ns.facebook.com. 5 AAAA 2a03:2880:f1fd:c:face:b00c:0:35
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0