Facebook has revealed it was the target of hackers last month, but insists no information stored about its millions of users was compromised during the attack.
According to Facebook's security blog the company discovered in January that its system had been targeted in a sophisticated attack.
It happened when a handful of the company's employees visited the website of a developer, which led to malware, or malicious software, being installed on their laptops.
Facebook says it has fixed the infected machines and is investigating. The company also says it was not the only company targeted in the attacks but was one of the first to discover it.