The UK is free to look into the Facebook experiment and to pursue the case. That alone doesn't make Facebook guilty of anything. They didn't go to a court of law, Facebook wasn't proven guilty of breaking the UK Data Protection Act, they're just investigating whether proper precautions were taken.
Facebook should obviously be bound to the same laws as everyone else. Which human experimentation laws did they break? Users registered on Facebook and agreed to the terms of service, and how Facebook will choose which stories they view, and analyze their response.
You did not answer my other question. Here is a more direct question.
Should Facebook be exempt from laws on human experimentation?