I did found a duplicated question about this topic right here What's the IP address range of Facebook's Open Graph crawler?
But just to verify the IP address range that the question's answer provided is accurate as of April 2012. Is that range of IP address still accurate for now as 2018?
The IP address range shown in the question is
31.13.24.0/21
31.13.64.0/18
66.220.144.0/20
69.63.176.0/20
69.171.224.0/19
74.119.76.0/22
103.4.96.0/22
173.252.64.0/18
204.15.20.0/22
In order for our "intranet" system which not allow any internet access has the facebook SSO feature, we need to configure on the firewall to allow the system access to the Facebook Graph API.