More details here
In a statement on its blog, Facebook apologised for the brief downtime.
"For a brief period of time, some internal prototypes were made public to a number of people externally." read the message. "As a result, we took the site down for a few minutes. It's back up, and we apologise for the inconvenience."
The BBC understands that the problem arose when code for a number of new products was exposed to the public before they were ready to be launched. This included a feature called "memories" that indexed what people have done on Facebook chronologically.