Or people who rely on Facebook for shared login...that must be down as well.

You're telling me--of course this has to happen the day before client review of a major Facebook Connect site whose only authentication is via Facebook Accounts. We had discussed potentially having two types of accounts (Facebook- and email-based), but having two parallel authentication methods just seemed clunky. I guess this is the price we pay for trying to keep things simple...

Good point. We recently had a client set their "signup" to FB login by default and I cringed.

Great, simple services but the macro effect that this is going to have on commerce today is crazy.

Seeing as Facebook login lets you easily collect email addresses these days you could always have a page that says, facebook is down click here to get an email confirmation link to login or something similar.

This must really be hurting Quora.

Luckily, they added Twitter authentication as well. But still, now a large portion of their site cannot login because of Facebook.

Well frankly, If you rely on some other business for such critical functionality, you deserve to lose.

Unfortunately its difficult to avoid relying on other businesses for crucial systems.

Example: Payment Processing, try building a business, especially an online one, without depending on Paypal, or at least Mastercard/Visa.

Sure you can reduce it by say not having FB Connect as the only log-on option but the simple fact is that you will almost always have to depend on other businesses, the best you can do is diversify and hope for the best.

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.

If you have a few options, then you don't rely on a single company, do you?

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.

for like buttons you should iframe them (same for anything from a third-party service)

facebook uptime has been good overall, considering they doubled in size in 12 months to 550m users

Even iframed, you end up with an ugly "service unavailable" blot of text.

Do better systems exist? A few hours downtime in several years is still better than most brick-and-mortar stores, I think.