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.
"We believe we've identified the root cause and taken steps to address the issue, although our investigation is ongoing and we'll continue to make security improvements," D'Angelo said.