In case you thought this was functionally simple...
At least one of the complications is that large-scale services such as social networks will store information (or copies thereof) as locally as possible -- either in part or in whole.
What happens when Brussels Bill and California Carl become Facebook friends? I don't know FB's data architecture, but I can guess that at least some of Bill's data is going to be replicated in California, while some of Carl's data will be replicated in Brussels. When the cops come knocking in California, the server should cough up only data on Carl, while keeping schtum on Bill?
On a much smaller scale, what would this mean for a small business owner who has customers or clients worldwide? Whether I'm using a cloud CRM or a desktop database, I am bound to be breaking the law.
Even under Safe Harbour, this was problematical, insofar as an undertaking to uphold _either_ US or Euro standard would place me into averred conflict with the other -- and we can thank Schrems for forcing the conflict into the open.