Tumblr has grown to be one of the 50 biggest sites in the world with over a billion page views a month. They've done this with (iirc) a fairly small team.

Sure they've got growing pains. It saddens me that some here are labeling them "morons". The problems they're now facing aren't trivial. How they handle them will really be the test,

Twitter had huge stability problems. They got over them. Even Google has had serious downtime (not since 2001 however).

It's not whether or not you make mistakes that matters. It's how you deal with them when you do.