Max Kelly, the chief security officer at Facebook, said the attack was aimed at an individual blogger, known only as Cyxymu - an outspoken critic of Russia's occupation of South Ossetia in Georgia last year.
Twitter was down for several hours yesterday, while Facebook and LiveJournal were slowed to a crawl due to an orchestrated denial of service attack. Google also reported that some of its pages were affected.
A similar attack hobbled the government websites in South Korea and the US earlier in July - during the attacks tens of thousands of highjacked computers simultaneously log on to a website, overwhelming its servers with a flood of traffic.
Kelly told CNet news: "It was a simultaneous attack across a number of properties targeting [Cyxymu] to keep his voice from being heard.
"We're actively investigating the source of the attacks and we hope to be able to find out the individuals involved in the back end and to take action against them if we can."
The attack came on the anniversary of the fighting between Russia and Georgia in South Ossetia, the breakaway republic recognised by Russia along with Abkhazia. At the time, similar internet attacks were directed at both countries, with Russian sympathisers attacking the Georgian government website and vice versa.
Thousands of spam messages were sent out from Twitter and Facebook mentioning the name Cyxymu with links to his LiveJournal blog, according to the blogger. However, he was not able to pinpoint who was behind the strike - one of the largest and well coordinated denial of service attacks to date - but he told the Guardian he suspected Russia.
"Maybe it was carried out by ordinary hackers but I'm certain the order came from the Russian government," said Cyxymu, whose name is a latinised version of the Russian spelling of Sukhumi, the capital of Georgia's other breakaway republic, Abkhazia.
He told the Guardian: "An attack on such a scale that affected three worldwide services with numerous servers could only be organised by someone with huge resources."
Kelly also would not speculate who was behind the attack, but said: "You have to ask who would benefit the most from doing this and think about what those people are doing and the disregard for the rest of the users and the internet."
Service to all websites has since been restored, but not before bloggers took to the internet in numbers to lament Twitter's history of spotty service.
The attack is the second high-profile failure of Twitter's security team. Last month, a number of classified documents were stolen by an anonymous hacker and handed over to the blog TechCrunch.
Twitter, along with Facebook, is a regular vehicle for spam and phishing attacks, such as the Koobface virus which infected thousands of Facebook accounts earlier this year.
Spammers are also taking advantage of Twitter's need for URL shorteners, where often malicious web addresses are disguised as "blind" links.
Twitter's Biz Stone wrote on the company blog yesterday: "Twitter has been working closely with other companies and services affected by what appears to be a single, massively coordinated attack. As to the motivation behind this event, we prefer not to speculate."