After declining about US$20 billion (NZ$30b) in 2018, Mark Zuckerberg's fortune is bouncing back.

Facebook shares jumped Thursday in New York trading - increasing his net worth by about US$4.6 billion (NZ$6.94b) - after the social-networking firm reported first-quarter earnings Wednesday night that won near-unanimous praise from Wall Street analysts. That brings his year-to-date advance to US$24 billion, the most of anyone on the Bloomberg Billionaires Index.

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Zuckerberg's gain tops that of Bernard Arnault, who has seen his fortune increase US$23 billion in 2019. Jeff Bezos would have topped both except for the finalization of his divorce from MacKenzie Bezos, which cut his net worth by US$36 billion this month and wiped out his 2019 gains.

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Facebook shares were up 7.2 per cent to US$195.78 at 9:55 a.m. and have climbed 49 per cent this year.

This comes despite the fact that Ireland's privacy regulator says it's investigating Facebook over the social media giant's recent revelation that it had left hundreds of millions of user passwords exposed.

The Irish Data Protection Commission said Thursday that it opened a statutory inquiry this week into Facebook after the company notified it about the security lapse.

The company said last month that it had inadvertently stored passwords for users of Facebook, Facebook Lite and Instagram in plain text, making it possible for employees to search them.

Facebook said it is working with the commission on the inquiry.

"There is no evidence that these internally stored passwords were abused or improperly accessed," the company said in a statement.

Facebook has said it fixed the problem and planned to notify millions of Facebook and Instagram users and "hundreds of millions" of users of Facebook Lite, which is primarily used in developing countries.

It's the latest regulatory headache for Facebook and CEO Mark Zuckerberg, who has promised to turn the company into a "privacy-focused platform."

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The Irish commission, which is Facebook's lead privacy regulator for Europe, already has 10 other investigations under way into the company and its subsidiaries over whether it's complying with European data protection rules.

Facebook said Wednesday it expects to be fined at least US$3 billion by the U.S. Federal Trade Commission, which is investigating whether the social network violated users' privacy. It also faces other probes in the U.S. and Europe, including in Belgium and Germany.

- Washington Post and AP