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For users, this meant no tweeting about lunch plans, the weather ... or the fact that Twitter was down.
"We are currently defending against a Denial of Service attack on this otherwise happy Thursday morning. Attacks such as this are malicious efforts orchestrated to disrupt and make unavailable services such as online banks, credit card payment gateways, and in this case, Twitter for intended customers or users. As I said, we are defending against this attack now and we'll continue to update our status blog as we continue to defend and later investigate."
Get more updates on Twitter's status blog.
The site still had lingering access problems midday, though there were reports of it functioning at least intermittently.
Facebook also was unavailable for a little while, but users are now able to log in. The company released the following statement:
"Earlier this morning, we encountered issues within our network that resulted in a short period of degraded site experience for some visitors. No user data was at risk and the matter is now resolved for the majority of users. We're monitoring the situation to ensure that users continue to have the fast and reliable experience they've come to expect from Facebook."
There are reports that LiveJournal also had some issues.
Earlier this week, Gawker Media was also attacked. In a blog post, Gawker said Tuesday it was attacked by "dastardly hackers," leading to server problems that caused network-wide outages Sunday and Monday. It was not immediately clear whether those attacks were related to Twitter's.