You might try connecting your computer directly to the modem with a wire, verifying that it doesn’t work (maybe try some of the others as well) to connect to facebook, then call your ISP to ask for help. You could even point out that you Googled the problem and found lots of other people on the same ISP having the same problem. Even their FB (I know) page is full of people complaining that they can’t connect.
I wonder if they’re throttling their service.
I would start with their tech support, just be nice and calm with them and see if you can get anywhere. If not then escalate. If you have other options and you’re willing to risk them dropping you, you can report them to the BBB or the FCC for blocking you from Facebook. There’s clearly enough other people out there with the problem that it’s not just you or your equipment, but tech support will start with the "I don’t know, reset your computer/router/modem/downforeveryoneorjustme/wait a few hours/try this try that/go to some other sites/it’s probably just your computer/I’ve never heard of this before/etc stuff first and you’ll have to play along. But I’d keep sticking with “Why am I paying for this if I can’t get to the sites I want to go to, can I at least get a discount until my service is resumed?”