It’s thought around 135 million people were unable to log on yesterday evening (23 September).
The problems started at around 7pm (BST) and lasted for two-and-a-half hours.
facebook blamed the problem on a software flaw which caused “huge disruption” to the site which has 500 million users worldwide.
That meant the site had to be shut down while engineers fixed the problem.
Millions of people posted comments about the shutdown on blogs and other sites like Twitter.
One of facebook‘s senior engineers Robert Johnson apologised to everyone who couldn’t log on.
In a statement on his blog he said: “The key flaw that caused this outage to be so severe was an unfortunate handling of an error condition.
“An automated system [to fix the problem] ended up causing more damage than it fixed.”
For many people who couldn’t log on that meant they were greeted by the message “Network Error (dns_server_failure)”.
Robert Johnson added: “Once the database had recovered and the root cause had been fixed, we slowly allowed more people back onto the site.”