Facebook said it fell victim to a "sophisticated attack" last month but has not found evidence that any user data was compromised.
The Menlo Park, Calif., company said the attack occurred after some of its employees visited a mobile developer's website that had been compromised. A malicious code hosted on the hacked website made it possible for malware to be installed on the employees' laptops.
"As soon as we discovered the presence of the malware, we remediated all infected machines, informed law enforcement and began a significant investigation that continues to this day," the company announced Friday.
According to tech news website Ars Technica, Facebook was hacked with a previously unseen type of attack using Java, a platform used for developing software.