Sarver also cautioned that the attack is ongoing and that its intensity and nature have changed several times since it started on Thursday, and as such Twitter is having to modify its defensive strategy along the way.

Twitter isn't alone in dealing with the attacks Friday afternoon. Google's Blogger got hit as well, according to a Google spokesman. "A small percentage of Blogger users have experienced error messages this afternoon as the result of what appears to be an ongoing distributed denial of service attack aimed at multiple services across the web," the Google spokesman said via e-mail.

"Google has a variety of systems in place to help counteract these types of attacks, and we believe the majority of affected users can now access their blogs. We're continuing to work to minimize the impact to affected Blogger users. No other Google products have been affected," the spokesman said.

On Thursday, a portion of the service that redirects third-party-owned URLs to Google Sites or the Blogger service also was affected for about an hour by the DOS attack, a source told IDG News Service.

Facebook and LiveJournal also got hit on Thursday. According to news reports and information from companies affected, the attacks appear directed at silencing a blogger in the country of Georgia who has been critical of Russia's actions and policies toward that neighboring country.