They completed five years recently. In a blog post to mark the occasion, one of their teams spoke about what they called "The Great Migration, the Winter of 2011". This is a reference to how they made several significant changes to their systems (without affecting their users) in order to prevent sightings of a certain non-bird.
Who are they and what did they not want people to see?
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Twitter. They use the "Fail Whale" icon when they experience problems (usually an overload), and made changes to accomodate the increasing use of the social networking site.