San Francisco, March 14 -- Facebook appeared to be back in operation Thursday after a massive outage affecting its applications around the world, creating fresh woes for the social network leader under fire for its handling of sensitive user data.

The outage affected users for some 12 hours in most areas of the world, with the biggest impact in North America and Europe, according to the tracking website downdetector.com, which suggested that most Facebook users were back to normal.

Facebook, however, offered little information on the extent or cause of the outage, which is believed to be the worst ever for the internet giant that reaches an estimated 2.7 billion people with its core social network, Instagram and messaging applications....