When you choose to delete your information after that period, we begin this process that we call active deletion. That sends a message to the various places on Facebook that store information about you so that we can provide service to you. What happens is the content is deleted or logs that have identifying information are removed. We may keep logs that are anonymous to you beyond that time, but the idea is generally that the information will get deleted. No system is perfect, but we do everything we reasonably can to make sure we honour that commitment.
On November 27th, 2012. See this statement in context.