Surprising that it's taking so long to come back, so I guess their IT department can expect to be eviscerated in the near future, unless the issue turns out to be entirely outside their control.
Their message about the situation - on Twitter, ho-ho! - is rather unsatisfactory: "We are aware some people are having trouble accessing our apps and products. We are working to get things back to normal as quickly as possible and apologise for any inconvenience."
"some" people? "having trouble"? Really, Facebook?
If it's a DNS problem, does that mean it's still accessible by using their IP address?