Facebook announced late this afternoon that its engineers were working to fix a problem causing site trouble for users.
In a posting on the its site, the company reported: "We are working as quickly as possible to fix a network issue that is causing the site to run slowly or time out today for some people. We apologise for any inconvenience and appreciate your patience." The announcement was posted at about 5pm EST.
Facebook has not yet made any updates about the problem, and users have been quick to comment on the post.
"Yeah... I'm having problems with things showing up when I post them, and messages that I've already read showing up as unread," wrote one user, Rachel Luczkowski.
Sharon Adams wrote, "Yes, it has been incredibly slow! Not been receiving my Inbox Messages until 24 hours after friends posted them!"
And as users usually do when Facebook is having problems, they vented their frustrations on Twitter.
On Twitter, FacelessOx tweeted, "Is it just me, or is Facebook messing up and glitching big time tonight?" And portiaalex wrote, "Who broke Facebook?"
Facebook has quickly become a social networking phenomenon.
The site, which has more than 400 million users, hit a major milestone last month. For the first time, Facebook replaced Google as the most visited website in the US for a full week.
Facebook was the most-visited website in the country for the week ending 13 March, according to Hitwise, an online traffic monitor.
While Google returned to the top spot, Hitwise noted that grabbing that position for a whole week was a major milestone for Facebook.