The biggest difference between the last time S3 crashed and this time, in my observation, is that there was a much, much bigger chain reaction this time around. Last time, I knew of only a few companies using S3, like photo hosting site SmugMug, and startups that offer online backup services using their own interfaces on the front-end and Amazon’s hardware infrastructure on the back-end.

This time, not only were those types of Web 2.0 companies affected, but much bigger fish also felt the sting: no less than Web 2.0 microblogging phenom Twitter and some iPhone applications crashed along with S3.

The last Amazon outage was attributed to “growing pains” as the service gained popularity. I’d imagine adding popular apps like Twitter and the iPhone constituted another wave of painful growth. This is a new medium, and users of very new storage media accept some level of risk. But two major outages in six months is obviously raising some questions.

“Skype has crashed and stopped responding, Twitter, Tumblr and other major websites are barely working, most aren’t displaying images, widgets or static material that was outsourced to Amazon S3 services,” reported blogger LinkFog as the outage occurred. “It’s kinda funny how this goes against the very nature of the web, in each networks are interconnected in several ways to ensure that a major breakdown won’t happen.”

Others, like a blogger at Web Worker Daily, were not happy with Amazon’s SLAs:

Amazon does offer an SLA for the S3 service, guaranteeing 99.9% uptime or part of your money back. With .1% of a month being around 45 minutes, that means they owe people money. The requirements for claiming a refund, though, are onerous enough that no one except large users will bother (hey, Amazon, how about an automatic refund when you know your servers are down?).

Recent reports suggest that this is actually what will happen.

Clearly it’s not a major disaster for people not to be able to Twitter for a few hours. But when it comes to things like the backup services attached to S3, it might be time for people to rethink whether one cloud back-end is the same as another. Amazon’s appeal is that it’s cheap and relatively unrestricted for Web developers–but I hope the backup companies basing their hardware infrastructure on S3 at least inform their end users what the back end is, so they can make an informed decision about service provider reliability.