Facebook was hacked, but says user data not affected

SAN FRANCISCO -- Facebook was hacked.

Intruders recently infiltrated the systems running the world's largest online social network but did not steal any sensitive information about Facebook's more than 1 billion users, according to a blog posting yesterday by the company's security team.

It's the latest breach to expose the digital cracks in a society and economy that is storing an ever-growing volume of personal and business data online.

Facebook, based in Menlo Park, Calif., serves as the gatekeeper for billions of photos and messages that get posted each month.

This time, at least, that material didn't get swept up in the digital break-in that Facebook said it discovered last month. The company didn't say why it waited until the afternoon before a holiday weekend to inform its users about the hack.

It was a sophisticated attack that also hit other companies, according to Facebook, which didn't identify the targets.

"We are working continuously and closely with our own internal engineering teams, with security teams at other companies, and with law enforcement authorities to learn everything we can about the attack, and how to prevent similar incidents in the future," Facebook wrote on the blog.

Facebook said the security lapse was traced to a handful of employees who visited a mobile software developer's website that had been compromised, which led to malware being installed on the workers' laptops. The PCs were infected even though they were supposed to be protected by the latest anti-virus software and were equipped with other up-to-date protection.