The information that has been compromised, Quora said includes, "account information, e.g.name, email address, encrypted (hashed) password, data imported from linked networks when authorized by users".
"Public content and actions, e.g. questions, answers, comments, upvotes [as well as] non-public content and actions, e.g. answer requests, downvotes, direct messages".
Quora's post said it will log out all affected users, and push a password reset.
"This is one of the most significant data breaches in history given the size - about 500 million people are affected - and the sensitivity of the personal information that was stolen", said CreditCards.com industry analyst Ted Rossman. In June 2016, hacking group OurMine broke into Google CEO Sundar Pichai's Quora account.
In a statement, Quora said the situation had been "contained".
"We're still investigating the precise causes and in addition to the work being conducted by our internal security teams, we have retained a leading digital forensics and security firm to assist us".
If you think you have a Quora account, the best thing to do is change your password.
Quora said it's in the process of notifying all users who it believes were impacted by the hack.
Presumably, Quora power users are included in this breach, and there have certainly been a few celebrities who've used the platform over the years, like the 44th president of the United States.
The site has already taken steps to log out all Quora users who may have been affected.
Quora relies on the website traffic that comes from people searching questions in their browsers. The company has also notified law enforcement.
For users of Quora, the company advised that they change their password - if they were using their Quora password across multiple services.
If you want to simply delete your Quora account, visit your account privacy settings and choose "Delete Account".