There will be an option on the router's web configuration page called URL blocking or web filtering. Your can unblock it from there. For accessing the router's web configuration page you have to type the default gateway on the brower's address bar. Your can your IE or Firefox.
Steps to find the default gateway
Start->Control panel->network and internet connections->network connection-> Right click Local area connection-> Status->Support
This will show you the default gateway something like 192.168.1.1
The open Internet explorer type 192.168.1.1 on address bar and unblock the facebook