New York (CNN Business) Question-and-answer website Quora warned late Monday that hackers gained access to the personal data of as many as 100 million of its users.

Quora discovered on Friday that one of its systems had been hacked by "a malicious third party," CEO Adam D'Angelo said in a blog post

The compromised information includes users' names, email addresses and encrypted passwords as well as data from social networks like Facebook ( FB ) and Twitter ( TWTR ) if people chose to link them to their Quora accounts.

The hackers also obtained details about users' activity on Quora, such as questions, answers, upvotes and downvotes. But anonymously written questions and answers were not affected by the breach.

"The overwhelming majority of the content accessed was already public on Quora, but the compromise of account and other private information is serious," D'Angelo said.