Reasons Tumblr went down
wellbebackshortly.com
wellbebackshortly.com
With that being said, they didn't take it lightly:
"know that this is absolutely unacceptable to our team, and unacceptable for a platform determined to be the best place in the world for your creative expression" [2]
I mean, what else can they say? Refund the customer's money? They've already done that.
[1] http://www.quantcast.com/profile/trafficGraph?wunit=wtpub%3A...
If your business is providing hosting for content or providing a service that is expected to be ubiquitously available then QoS is a significant part of your business whether your service is nominally free or not.
Edit: Whether your revenue comes from users or from advertisers the reliability and quality of your service matters. If your userbase decides to abandon you then you are screwed regardless of whether they are your customers or your product.
There's no excuse for downtime, but if there were, this would certainly be one.
Oh OK, I'll wait...gone.