Facebook was unavailable for a number of users mostly across Europe on Wednesday due to "technical difficulties".
Users reported problems accessing Facebook.com in many countries, including the UK, for about two hours.
The social network's smartphone and tablet apps were also affected, users on Twitter reported.
Facebook apologised for the incident and said the issue was now resolved.
"Today we experienced technical difficulties causing the site to be unavailable for a number of users in Europe," the company said in a statement.
"The issue has been resolved and everyone should now have access to Facebook. We apologise for any inconvenience."
More isolated accounts of inaccessibility were posted by users in further afield places such as Pakistan and South Korea.
The site has about 850 million users worldwide and many took to Twitter to share information about the down time, with #facebookdown quickly becoming a trending topic on the microblogging site.
Most took a humorous view, with user @Purple_Cow tweeting: "#Facebook is #down, expect a surge in productivity in offices everywhere today."
However, some website owners who make use of the Facebook Connect service - which allows people to log into external sites by using their Facebook account - were left frustrated by the technical issues.
"Facebook two hours down time this morning may make us rethink our login strategy," remarked Tariq Krim, founder of cloud-based service Jolicloud and news aggregator Netvibes.
Launched in February 2004, last month Facebook announced plans to float on the stock market. It seeks to raise $5bn (£3.16bn) from the move, making it the biggest sale of shares by an internet company