From TechCrunch:

Yesterday, though, Facebook announced something very unsettling. A “sophisticated attack” uploaded malware onto the computers of several Facebook engineers when they visited a hacked mobile developer site. Facebook quickly quarantined and scrubbed the devices, called the police, and kicked off an investigation. So far, there’s been no evidence that any user data had been compromised. Perhaps the attackers were after Facebook’s trade secrets or information about partners. Regardless, it was a very close call.