Twitter.com, the site devoted to pithy posts, was brought down on Thursday by a concerted attack by hackers. Facebook, another popular social networking site, also appears to be down.
Twitter, which limits postings, called Tweets, to 140 characters, is back up now -- and the site has promised to fend off the attack and take the fight to the enemy.
On its official blog, co-founder Biz Stone had put up this message: "On this otherwise happy Thursday morning, Twitter is the target of a denial of service attack. 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. We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate."
Twitter was famously unreliable as a service for a long time but, until this attack, has been reasonably stable.
A denial of service attack often involves bombarding the site's servers with connections, so that it goes down under the pressure.
Facebook is still to address its status on its official blog.
Image: Evan Williams, CEO of Twitter. Photograph: Rick Wilking/Reuters