In a statement, the company apologised but did not explain what went wrong.
Some of Facebook's 500 million users worldwide were shut out of the site entirely; others experienced sluggish page-load times. Problems were spotted in the US, Europe and South America.
The issues started at about 2:05 p.m. EDT, said Vik Chaudhary, a vice president at website monitoring company Keynote Systems.
People were having problems accessing the site for about four hours, he said. Keynote, which monitors website traffic in the US, found that over the course of the four hours, Facebook was unavailable to 22 percent of those who tried to access it.
Normally the site's accessibility hovers close to 100 percent, Chaudhary said.
Facebook users also experienced intermittent problems accessing the site yesterday, but spokeswoman Meredith Chin said the problems todaywere unrelated.
Chaudhary said he did not know for sure what caused the Facebook outage, but said it might have been a problem with Facebook's "content delivery network." Such a network helps make sure people around the world can access websites without lags in speed.
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.
