Facebook noted on its site this afternoon that it too was fending off a distributed denial-of-service attack that was slowing its site. Unlike Twitter, this morning, Facebook remained online.

"You may have had trouble accessing Facebook earlier today because of network issues related to an apparent distributed denial-of-service attack," the company wrote. "We have restored full access for most people. We'll keep monitoring the situation to make sure you have the reliable experience you expect from us."

Web site performance monitor AlertSite reported that Twitter's site wasn't back 100% until 2 p.m. EDT. AlertSite also noted that Facebook appeared to suffer little more than a few sporadic errors. Facebook's availability was at 97% at 10 a.m. when the attack was underway, and it was up to 100% availability soon after that.

While there's little more than online chatter and guesswork about the origins of the attacks, security analysts say the incident raises red flags that two giant Internet companies, like and would be hit in the same assault.

Randy Abrams, director of technical education at ESET, an IT security company based in Bratislava, Slovakia, said his best guess is that a was offering a demonstration of the power of his botnet to a potential client with a major target in mind.