Paul,
I am not sure that blocking it on the home router will be solution.
Internet access is available everywhere and the methods that can be used to gain access to FB are considerable.
All she needs to do is to log into any friends home network or tether into a bluetooth enabled phone of her friends to log in elsewhere rather than do it at home.
Cafes, Hungry Jacks etc etc all provide free internet services to their customers.
Being able to block all of Facebooks IP addresses on the computer will help, but then all she needs to do is log in via a thin client to her friends computer across the net (any friend at any number of IP addresses !) and log into her FB account via their computer.
Then there is a plethora of Web Proxy sites that do it all for you via browser access.
You would need to remove all web browsers and programs with web browsing capability to do that - is that going to be possible or even work ?
She could just install them again or run another one off a USB stick bought at Officeworks or Coles for $4!
Depending on where she is (including school) they probably haven't blocked HTTPS - secure browsing so she can get to FB in that way - Facebook provided this recently.
Failing that she can ping Facebook and get a local IP address and log into that with her browser - eg instead of using
www.facebook.com, she doesn't use that - Facebook has many IP addresses - you may not be able to block all of them.
I get 66.220.149.11 when I ping facebook from the DNS server - that may or may not work.
Next step is get her own phone and log in using that.
Use her friends computer or mobile phone and use that etc etc
Failing that, she gets everything remailed or twittered etc etc and communicate with people that way or even use a girlfriend as a proxy
Its not a matter that she doesnt have the technical expertise to do it - she only needs to look it up from any of the 156,789,643 (and counting kids) who use it and want to make sure they all can continue to use it !
They will find 10 different ways and publish how to do it in three easy steps that any fool can emulate.
What I am really trying to say is that I dont think you will be able to successfully modify your daughter's behaviour, by trying to restrict her access, the problem is more complicated than that.
You could try threatening to restrict access if her behaviour doesnt improve though - that might work.
Or maybe change the basis of other privileges being granted if FB behaviour doesnt improve.
Prohibition doesnt always work and in this case I cannot see how you practically implement it unless she is reasonable and honest with you.
Hope it works out for you.
Cheers
Rally
even just loosing net access at home for a week will make her never do it again 
I know it's a horrible thing to even think, but there are times when that seems like a vey sensible option.
