When parents saw their children run outside to play, and productivity shot up at offices around the world, people knew something was wrong.
Yes, Facebook had gone down.
For a couple of hours Thursday, many of the social network's 550 million users found they could not view one another's wedding photos, share stories from TheOnion.com or chat about how bored they were. They couldn't even let their friends know, via Facebook, that Facebook was down.
During the outage, the company said it was "experiencing some site issues causing Facebook to be slow or unavailable for some users."
"We are working to resolve this issue as quickly as possible," a representative said in an e-mail.
The problems eased about two hours later, when Facebook said the issue had been resolved and it apologized for any inconvenience. But some users said they were still having problems Thursday afternoon.
The downtime was relatively rare for Facebook. There was a brief glitch in April, as well as some slowness issues earlier this week.
A company official said the outage was not the result of an attack by a hacker.
The problems come at a busy time for Facebook, which is awaiting the release next week of "The Social Network," a movie that portrays founder and Chief Executive Mark Zuckerberg in an unflattering light. (A Times story in Friday's paper covers the uneasy relationship between the company and the movie.)
On Wednesday, news outlets reported that Zuckerberg was donating $100 million to the Newark, N.J., school district. The company's 26-year-old founder rose on Forbes' list of richest people, with the magazine estimating his worth at $6.9 billion, a number that would put him above Apple Inc. chief Steve Jobs, who clocks in at $6.1 billion.