Facebook announced last month that its corporate infrastructure had been compromised through a watering-hole attack against several of its employees. This was not the first major problem that the Facebook incident response team had handled. The previous two incidents involved a complete compromise of the Facebook network and the other involving an engineer getting owned via a zero day and the attackers inserting a backdoor into Facebook s production code. The attackers apparently had been in the network for some time and were angered by Facebook’s decision in January 2012 to publish information on the crew behind the Koobface worm.
Source: https://threatpost.com/how-facebook-prepared-be-hacked-030813/77602/