Facebook said on Friday that it been the target of a series of attacks by an unidentified hacker group, but it had found no evidence that user data was compromised."Last month, Facebook security discovered that our systems had been targeted in a sophisticated attack," the company said in a blog post. "The attack occurred when a handful of employees visited a mobile developer website that was compromised."The social network, which says it has more than one billion active users worldwide, added: "Facebook was not alone in this attack. It is clear that others were attacked and infiltrated recently as well."