We use ATLAS data to graph Facebook (AS32934) traffic with 80 ISPs around the world between 5pm September 22 and 5pm EDT today. You can see Facebook traffic plummet around 1:30pm and return shortly after 4pm. From a quick glance at the data, the outage appears to be global (impacting all of the 80 ISPs).
We have no information on the root cause (no sign of obvious BGP instability or DDoS).
Lots of speculation on twitter.
UPDATE 8:30pm Sept 23: Facebook explains this was an internal configuration management problem.