NEW YORK A hacker attack shut down Twitter on Thursday morning, and Facebook also said it was looking into possible site problems.
Twitter said in its status blog Thursday that it was defending against a denial-of-service attack, in which hackers command scores of computers to a single site at the same time, preventing legitimate traffic from getting through.
For users of the fast-growing messaging service, the outage means no tweeting about lunch plans, the weather or the fact that Twitter is down.
Facebook also seemed to be experiencing problems, at least intermittently.
Facebook spokeswoman Brandee Barker said the company was looking into it and would have an update as soon as possible.
The Twitter outage began at about 9 a.m., said Ken Godskind, chief strategy officer at Web performance monitoring company AlertSite. The site still had access problems late morning.