The social media giant said the breach, which exploited a vulnerability in its code to take over user accounts using access tokens was discovered on Tuesday.

It has since been resolved but bosses have admitted they weren’t sure if user details are safe.

Facebook said it logged out around 90m users as a safety precaution, while around 50m were believed to be affected.

Facebook announced the hacking on Friday. Picture: Dominic Lipinski/PA Wire

If your account was one of those hacked, you may notice that your email or password has been changed.

Facebook says 50 million users affected by '˜security breach'

You may also notice if your name or birthday has been changed, friend requests have been sent to strangers, messages sent you didn’t write and posts created that you didn’t make.

If this is the case, you’ll be prompted to log back into your account as well as any other apps you use with your Facebook login. You’ll then get a notification at the top of your news feed telling you what has happened.

Guy Rosen, Facebook’s vice president of product management wrote in a blog post: “Since we’ve only just started our investigation, we have yet to determine whether these accounts were misused or any information accessed. We also don’t know who’s behind these attacks or where they’re based. “People’s privacy and security is incredibly important, and we’re sorry this happened. It’s why we’ve taken immediate action to secure these accounts and let users know what happened.

“Users are not required to change their passwords, but people looking to log out of all the various places they’re logged into their accounts can do so with a single click through the Security and Login section under the settings menu.”

Meanwhile, an investigation into the sale of personal data on the dark web from price comparison experts Money Guru has uncovered that Facebook logins are sold on the dark web for just £3.

Research on several dark web marketplaces and uncovered that criminals can buy Facebook logins for just £3, and email logins for as little as £2.10. During their research into the value of logins for 26 of the most commonly used accounts available on the dark web, Money Guru found that hackers can purchase the majority of someone’s online life for £744.30.

This can include usernames, passwords, email addresses and any personal details associated with your account including name, address and phone numbers.

And it takes less than 10 minutes to create an anonymous account, select someone’s data from the marketplace and reach a payment screen.

A version of this story appeared on our sister site the I

Facebook Data Breach: How to tell if your account has been hacked