>>EDITED because I found out that this is even easier than I first realized<<
Assuming you have blocked Facebook, here's how a user can access Facebook:
Step 1) Use Google Chrome.
Step 2) Type facebook into the address bar
Step 3) Select "facebook login" from the drop-down list that appears
Step 4) Click the link and log in
Awful. This is true of the routers I've tried from Cisco, Netgear, and D-Link.
Try it and see.