Some Belkins support URL blocking. In the web interface, it will be somewhere like "Access Control" or "Schedule": in the former case, you link the rule to the MAC address of the individual computer. In the latter case, you'd create a rule (which would apply to all computers) blocking access to Facebook, and schedule it to run 24x7. It's a long time since I played with Belkin, though. The manual or Belkin's website should be able to help.