Facebook users around the world have reported problems updating their status or interacting with friends.
Some of the website's estimated 1.15 billion members were shown an an error message which read: "Couldn't update status. There was a problem updating your status. Please try again in a few minutes."
Some users also said they were unable to 'Like' posts, post comments or upload photos, although others experienced no difficulties using the website.
Hundreds of comments about the apparent service outage appeared on Twitter every few minutes.
A spokesman for Facebook said some users had reported difficulties using the social networking site, adding: "Facebook is looking into these reports."
Michael Allen, a director at Compuware Corporation, which tracks internet down time using its Outage Analyzer tool, said the apparent technical problems would have an "enormous impact" on millions of people.
He said at least 3,500 other websites had reportedly been affected, "although the actual number is likely to be far greater".
"This is because many other businesses and websites are connected to Facebook through the integration of its services," he said.
"For example, Facebook is used to enable people to login to many other sites and applications.
"Any organisation that is seeing errors or slowdowns with their own site or application should check to see if they are relying on Facebook services before they start fire fighting, as this might be the cause."
Down Right Now, which monitors the status of some of the internet's most popular services, warned Facebook users that disruption was "likely".
It began to receive widespread reports from people to struggling to use the website at around 12.50pm.
Founded in 2004 by Mark Zuckerburg, Facebook has around 700 million daily active users, according to its own figures.
It stores more than 100 petabytes (107 billion megabytes) of photos and videos at its three US data centres in Oregon, North Carolina and Iowa, as well as a fourth in Lulea, Sweden.
