Some 30 million user accounts were stolen in a huge hack of Facebook, the company has confirmed. The breach – the worst in the company's history – appears to have been carried out by an unnamed attacker, Facebook suggested. It said the FBI was "actively investigating" the hacker, and that it had "asked us not to discuss who may be behind this attack". The company had already announced that around 50 million users' data had been exposed when hackers stole login keys that allowed them to access profiles. But it wasn't clear how many of them had actually been used.