Facebook called it "the worst outage we've had in over four years" in a blog post published late Thursday.
The company blamed a software flaw that caused a cluster of its databases to be overwhelmed. Facebook said it had to shut down the entire site to stop traffic to the databases. The company estimated the problems persisted for about two and a half hours.
By evening, the site was again running smoothly, but some people affected by the problems were still cooling off.
Jennifer Sokolowsky, a freelance writer and editor in Seattle, was in the middle of contacting some Facebook friends about a possible job when the site slowed to a frustrating crawl.
"I realized I don't have any other way to contact them," she said. "If it's not working properly, that's when you realize how tapped in you are to it."
The problem started about 2:05 p.m. EDT, said Vik Chaudhary, a vice president at website monitoring company Keynote Systems. Over four hours, the site was unavailable to 22 percent of those who tried to access it, Keynote said. Normally, it's available close to 100 percent of the time.
Any stutter in Facebook's services is bound to cause a commotion on the Web. Its members now spend more time on the social network than they do even on Google, which owns the leading Web search engine, YouTube and other sites. In August, Facebook commanded 41.1 million minutes of U.S. surfers' time, compared with 39.8 million minutes spent on Google sites that month, according to research group comScore Inc.
For some, the technical problem upstaged news that Facebook CEO Mark Zuckerberg, 26, isto the struggling Newark public school district. The donation is being announced Friday on Oprah Winfrey's TV show.
Facebook is also under the microscope ahead of the Oct. 1 release of the movie "The Social Network," a fictionalized account of Facebook's founding that paints a less-than-flattering picture of the young CEO.
The Facebook outage came two days after a mischievous hack spread through Twitter, the short messaging site. That attack didn't shut Twitter down but it spread "tweets" of blocked-out text to people's accounts, causing pop-up windows to open on their computer screens.