The attack also affected Facebook, LiveJournal, Google's Blogger and possibly even YouTube, which has led to speculation that the target was an individual not a web site.
According to Facebook, what all these sites have in common is a user who is an anti-Russian blogger called Cyxymu from Tbilisi, the capital of Georgia.
This theory is given "credibility" by that fact that the DoS attacks coincide with the first anniversary of the start of last year's conflict between Russia and Georgia.
Facebook claims cross-service DoS attacks were all aimed at preventing Cyxymu communicating with his followers on this date.
Speculating about what caused the attack is probably more interesting but not nearly as important as the effects of the attack, particularly on business.
An increasing number of businesses are using Twitter, Facebook and other social media to engage with customers and partners.
UK cyber security experts have warned businesses against becoming too reliant on these services because of their vulnerability to attack.
There is little these sites can do against a well organised denial of service (DoS) attack, says Tony Dyhouse, director of the UK's Cyber Security Knowledge Transfer Network.
Twitter was downed by a DoS attack yesterday and took nearly three hours to restore services after it experienced connectivity problems.
Business and other users were unable to access the microblogging site for at least 90 minutes as Twitter scrambled to deal with the problem.