My Missing Past

I decided to download my Facebook History yesterday to see what I'd been posting two years ago. To do so, log in, go to account settings and it's at the bottom. facebook emails you a link, which you click on, and then enter your password to verify your identity.You then download the history, which in my case was nearly 16Mb. It was a dull read, made all the more interesting by the fact that nearly 6 months of posts are completely missing. What's going on? Is this yet another rolled-out feature that is not actually ready to be rolled out or is something more interesting going on? I'm guessing the former, but if Facebook can lose so much data, what happens when the authorities go knocking on their door with a warrant for someone's history to use as evidence in a court case and they can't produce it?