Twitter’s service went down around 9 a.m. EDT and was back up around noon, while Facebook’s site also experienced problems that may have been the result of a DoS attack. Twitter has become a favored target of attackers of late, as the site offers a huge user base and numerous attack vectors to exploit. The only communication from Twitter so far on the attack has been a series of status updates disclosing the attack and saying that the company was working to restore service. Twitter users regularly see the famous fail whale image telling them that the site is over capacity, but that condition usually lasts just a few minutes.
Source: https://threatpost.com/update-twitter-suffers-dos-attack-080609/73002/