Load balancing with single Domain name

[xewebow396]

I have we common scenario such as i have two different ISPs both are on same pfsense and one server is behind the LAN, Load balancing is done .

ISP1------>
               pfsense ------> LAN (server)
ISP2------>

I have single domain name which is pointed to both WANs. But I am confused and unable to understand that like google.com, facebook.com both has single IP against domain name. How can I do the load balancing of DNS so that when i ping domain or check up from nslookup it shows only one IP and one wan goes down other should be enable.

Looking for help. Thanks