(AP) - A hacker attack Thursday shut down thefast-growing messaging service Twitter for hours, while Facebookexperienced intermittent access problems.

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

The fact that a relatively common attack could disable such awell-known Web site shows just how young and vulnerable Twitterstill is, even as it quickly becomes a household name used bycelebrities, large corporations, small businesses and evenprotesters in Iran.

"Clearly they need a stronger infrastructure to be able tofight this kind of attack," said Graham Cluley, senior technologyconsultant at computer security firm Sophos. Twitter's tech supportteams, he added, "must be frankly out of breath" trying to keepup with the site's enormous growth.

For Twitter users, the outage meant no tweeting about lunchplans, the weather or the fact that Twitter is down.

"I had to Google search Twitter to find out what was going on,when normally my Twitter feed gives me all the breaking news Ineed," said Alison Koski, a New York public-relations manager. Sheadded she felt "completely lost" without Twitter.

The Twitter outage began at about 9 a.m. EDT and lasted a fewhours.

Facebook, whose users encountered intermittent problems Thursdaymorning, was also the subject of a denial-of-service attack, thoughit was not known whether the same hackers were involved. UnlikeTwitter, Facebook never became completely inaccessible. Facebooksaid no user information was at risk.