Twitter said in its status blog yesterday morning that it was "defending against a denial-of service attack," in which hackers command scores of computers to access a single site at the same time, preventing legitimate traffic from getting through.

Facebook and Twitter have become targets for attacks as their popularity soars. Hackers often target popular sites in an attempt to extort "protection money" from their owners or to make a name for themselves, said Joris Evers, a spokesman for security-software maker McAfee Inc.

"This could be for the notoriety of taking down a high-profile Web site like Twitter," Evers said in an interview from Santa Clara, Calif. "It could be for political reasons. Or it could be part of an extortion scam."

McAfee is investigating the Twitter attack, he said.

The fact that a relatively common attack could disable such a well-known Web site shows just how young and vulnerable Twitter still is, even as it quickly becomes a household name.

In Richmond, the Twitter outage was an unwelcome disruption to Kate Hall's daily routine.

"I'm a little bit lost. It's such a habit for me now," said Hall, founder of the online parenting magazine Richmondmom.com. "It's like your morning coffee -- you've got to have it."

Hall said she uses Twitter to keep up with news and the Richmond arts and social scenes.

"I really miss finding out what's going on," she said.

Unlike Twitter, Facebook never became completely inaccessible. Facebook said no user information was at risk. By early afternoon, both seemed to be functioning, though Twitter warned that as it recovers, "users will experience some longer load times and slowness."

LiveJournal, a 10-year-old online diary and blogging site that has waned in popularity in recent years, also was the subject of a denial-of-service attack that lasted about an hour yesterday morning, the company said.

In July, U.S. government and private company Web sites were attacked by unidentified hackers. Sites of the U.S. departments of State, Treasury and Transportation as well NYSE Euronext, the world's largest owner of stock exchanges, were affected.

Twitter, based in San Francisco, drew an estimated 17.6 million users in the U.S. in May, a more than 27-fold increase from a year earlier, according to research firm ComScore Inc. in Reston.

Source: Richman Times Dispatch