Facebook users have been warned to be vigilant by cyber security watchdogs after it emerged the tech giant had suffered a security breach affecting 50 million users.
In a post on the social network's news site, Facebook vice president of product management Guy Rosen said the breach had been discovered on Tuesday.
But executives waited until Friday to announce the news to users.
Mr Rosen said: "Our investigation is still in its early stages. But it's clear that attackers exploited a vulnerability in Facebook's code that impacted 'View As', a feature that lets people see what their own profile looks like to someone else.
"This allowed them to steal Facebook access tokens which they could then use to take over people's accounts.
"Access tokens are the equivalent of digital keys that keep people logged in to Facebook so they don't need to re-enter their password every time they use the app."
The firm later said the issue had arisen due to the combination of three distinct bugs which meant the hackers were able extract the access tokens of other users.
It was unclear whether any Irish users had been hit, but some reported getting a message when they tried to log in that said: "Recent activity may have affected your account's security, so we've locked it.
"We'll walk you through a few steps to confirm your identity and help you access your account."
Mr Rosen outlined the action Facebook had taken since the discovery, including fixing the issue and reporting it to law enforcement.
It also reset the access tokens of a further 40 million accounts that have been subject to a 'View As' look-up in the last year as a precaution.