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Quora question about that:

http://www.quora.com/How-does-Facebook-implement-feature-swi...

I suspect that if new features require storage changes, the changes are strictly additive and won't affect old features, or old features are modified to use the new storage setup.

With the amount of data Facebook has, they probably don't have the option of an "iterate over every row and change a thing" kind of migration.

No, but they do have an extremely fast and efficient tool for performing online schema changes to its huge MySQL deployment: https://www.facebook.com/notes/mysql-at-facebook/online-sche...

Eventually you'll have users in the same state, either because the push is not atomic or because the change is rolled out slowly. This means you have to build the change with this in mind regardless of the employee testing.