By Dan Whitworth
Newsbeat technology reporter
Social networking site Facebook says it has suffered the worst shutdown in four
years.
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.
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The key flaw that caused this outage to be so severe was an unfortunate
handling of an error condition
Facebook statement
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."