We pick b.root-servers.net, one of the 13 root servers (1), and ask it about zero.facepooke.com and learn that com is delegated to 13 name servers (2), so we ask h.gtld-servers.net where we learn that facepooke.com is delegated to two name servers, including ns3.above.com (3), so we ask that one and get the following answer:
All of the several thousand of domains that use the primary name server ns1.above.com use name server group 1 (example: pierlucasrl.com, fragbit.se and mountainexplorer.com ). Worth noting is that three quarter of the domains that use name server group 1 use the primary name server ns1.above.com.
The two name servers ns1 and ns2.above.com together
There are several thousand of domains that use name server group 1.
The four name servers ns1, ns2, ns3 and ns4.above.com together
There are several thousand of domains that use the four name servers ns1, ns2, ns3 and ns4.above.com together, hereafter referred to as "name server group 2".