Social networking site Facebook says it has suffered the worst shutdown in four years.

社交网站Facebook称其遭受了四年以来最严重的故障。

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（瑕疵，裂纹） which caused "huge disruption" to the site which has 500 million users worldwide.

That meant the site had to be shut down while engineers fixed¹ 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（中断） to be so severe was an unfortunate handling of an error condition.

"An automated² 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."