Exactly...people made this same argument a few years back when Amazon S3 went down for a few hours. Just have a few different options, and the chance that they will all be down at the same time is pretty slim.
Yes and No. If you have a few options than your entire business won't come grinding to a halt but if half of your customers choose to use FB Connect as their login option than they likely don't care that FB's down, or that people with email logins can still access your site. To 50% of your user-base your site has failed and so in that sense I would say that you still rely on FB for your business.