Your question seems to be conflating two different ways that VPN's are used:
1) Corporate VPN's are often used by companies to enable their employees to work remotely. When an employee is working remotely, he/she can connect to the company's network through a VPN. Then, it's as if the user is on the 'trusted' side of the firewall, and it's as if they are on the company's network. They can access other resources (e.g. network file servers, etc.) on the network. Also, any outside connections will appear to be coming from the network's public IP address. This is normally what people mean when they refer to a VPN integrated with a firewall.
2) VPN services allow users to route their network connection through the VPN service's network. User's typically use a VPN service to make their internet connection appear to be originating from an IP other than their own, for privacy reasons. Or, users can use a VPN service to 'bypass' blocks on outgoing connections from the network that they are on to certain services (like Facebook). The connection to the VPN is typically encrypted, so anyone/anything monitoring your traffic on your local network will have no way of knowing that sites you are accessing through the VPN.
So in your case, if you were on your company's network, and you wanted to access Facebook, you could try connecting to a VPN provider, and you may succeed in getting around your company's block on Facebook this way. This is basically (2), above. However, this is not the same way of using a VPN as described in (1) above, which is the case where a VPN is integrated with a firewall, typically used for allowing remote access to the network.