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.”

Facebook’s announcement follows recent cyber attacks on other prominent websites. Twitter, the microblogging social network, said this month that it had been hacked, and that approximately 250,000 user accounts were potentially compromised, with attackers gaining access to information including user names and email addresses.

Newspaper websites including The New York Times, The Washington Post, and The Wall Street Journal have also been infiltrated, according to the news organizations. Those attacks were attributed by the news organizations to Chinese hackers targeting their coverage of China.

(Reporting By Tim Reid)