Users in the south and south-east of England started experiencing the most difficulty.
But now users in the US, Brazil, Peru, Colombia, Mexico and parts of Europe have also reported they are unable to post on the two social media sites.
Both are displaying error messages once those in these areas attempt to use them.
A message on Facebook reads: "Oops... Something went wrong. We're working on getting it fixed as soon as we can."
It's unclear what the reason for the fault is but frantic users have ranted on Twitter this afternoon.