Are their APIs and Sign in with Facebook, also down? That's pretty messed up for all the sites that rely on their Sign in.
This is just one of the reasons you shouldn't build your business on someone else's platform - others include the possibility that they'll charge you for the service later, cut you out of a relationship with your own customers, shut you down for their own reasons, require you to use their services like a store to the exclusion of all others, copy your idea and crush you by giving it away for free, squeeze your margin…
If you want to take advantage of this market then there are ways to use Login with Facebook without being wholly dependent. Basically if you have full account management, but you allow third-party authentication that ties into that account, especially allowing multiple OAuth providers to be linked to a single of your internal accounts (eg. see how Stack Overflow works), you can significantly mitigate the downside.
The purist and old-school web head and open standards guy in me hates it, but you can't argue with the business case for it.