Twitter is down because of a denial-of-service attack and Facebook's been sluggish and unresponsive on Thursday.

We are defending against a denial-of-service attack, and will update status again shortly, blogged Twitter on its official blog.

Twitter site did restore for some time but as of 1:30 pm ET, the site is down again.

As for Facebook, the popular social-networking site did not release any official reason as to why the site is slow and at times, unresponsive.

Earlier this week on Monday, sites operated by Gawker Media crashed and it was later believed to be the result of a DDoS attack against The Consumerist blog, which is owned by Consumers Union but whose content is hosted by Gawker Media.