Twitter is crawling back to life this morning after a Denial-of-Service attack knocked the popular communications service offline for about two hours.  In a status report, the company says that the "site is back up, but we are continuing to defend against and recover from this attack."

Twitter has suffered outages before, but they were mostly tied to an inability to keep up with the rapid growth of the service. Earlier this week, comScore reported that the social media tool attracted 44.5 million visitors in June -- a 1,460 percent increase over the same month last year. 

As Twitter grows larger and attracts business users, outages such as these become much more critical.

Denial-of-Service attacks have brought down big Internet sites in the past, essentially flooding a company's servers with unwanted Internet traffic. Just last month, an attack that was reported to have originated in North Korea knocked out the U.S. Treasury and Federal Trade Commission Web sites, along with others.

That attack highlighted the importance of cybersecurity at the highest levels, with Philip Reitinger, deputy under secretary at the Homeland Security Department, telling the AP that it remains a critical national security issue.

"They're just zombies that are being used by some unseen third party to launch attacks against government and nongovernment Web sites," he told AP.

In 2000, a Canadian teenager nicknamed "MafiaBoy" instituted a denial-of-service attack on sites such as Amazon.com, eBay and Dell.  He was later sentenced to eight months in a youth detention center, with the BBC reporting that the outages caused an estimated $1.7 billion in damages.

UPDATE: At 8:40 a.m. Twitter was down for me, though I had no problems accessing the site around 8:15 a.m.

Update: Here's the latest message from Twitter's status blog. "As we recover, users will experience some longer load times and slowness. This includes timeouts to API clients. We’re working to get back to 100% as quickly as we can."

Update: Facebook and LiveJournal confirm that they too were hit by a denial-of-service attack, reports the The Wall Street Journal.