It sounds like a DNS (domain name server) problem to me.
When you type in an address such as www.facebook.com, the information from your PC is sent to your ISP's domain name server, which converts it into the 'real' (numeric) address of the site you're trying to access. Sometimes this fails to work, either because of a problem with the DNS itself or, more often, because of a problem in the way that your PC connects to it.
Try a test. Check that you still can't access www.facebook.com. (If you can the rest of this is rather pointless!). Then type this into your brower's addres bar
69.63.176.12
and click 'Go':
If using that numeric address takes you to Facebook (when the conventional route doesn't), it's definitely a DNS fault.
If you find that you've got a DNS problem, it might be worth checking with your ISP to see if the problem is at their end. (Ice.Maiden was having similar problems last night. She's with BT). Otherwise, you'll need to get hold of the primary and secondary DNS settings from your ISP. (They're just lists of numbers, like the numeric address I gave you above. They should be on the support pages of their website, or you can ask over the phone).
Then disconnect from the internet and go into your internet connection settings. Under 'Networking', look for 'TCP/IP Settings'. In there, you'll probably see a check mark alongside 'Server assigned name server addresses'. Click to move the check mark to alongside 'Specify name server addresses'. Then, in the relevant fields, enter the numbers for the primary and secondary domain name servers (that yo got from your ISP). Click 'OK' and 'OK' again. Reconnect to the internet and see if the problem has cleared.
Chris