Data wasn't transferred to the US
My problem with the legal argument is that the data was never transferred to the US. It has only ever existed in the US so it hasn't been transferred anywhere. When the user enters their details into Facebook, the data goes straight to the US and is never stored in Europe.
I totally agree that the MS issue is far more important, but we knew back in 2007 when the SWIFT scandal broke that the US could compel anyone in the US with access to data overseas to give them a copy. (http://www.theregister.co.uk/2007/02/16/swift_hm_treasury/)
Biting the hand that feeds IT
