Please provide a few more details about your ISP and modem/router equipment (make/model). Have you tried rebooting the modem and router? Have you tried connecting a computer directly to the modem?
If you have access to FB when connected directly to your modem, then you may need to check the settings on your router or perform a reset to defaults. If you still can't access FB with a direct connection, then it's possible it's on your ISP's side, and you would have to work with them to resolve the problem.
On our home/work network (a couple computers at home/small business) we can open any website...but all important Facebook. It does not matter the browser (Chrome, Safari, Firefox, IE...) or if it is a Mac or PC. This only started happening about a week ago. We are at a loss regarding what is going on. Additionally, FB will open for a couple seconds/minutes (but is slow and will not open pictures, etc.) like once a day, but then becomes unavailable again shortly after ("failed to open page" - not responding). We are thinking it has something to do with our network, but not sure how to resolve it.
Thank you for your feedback in advance.