Facebook has admitted that information of up to 87 million people, mostly in the US, may have been improperly shared with the British political consultancy firm Cambridge Analytica.

In a press conference on Wednesday, Facebook CEO Mark Zuckerberg said that they did not put out the 50 million number as previously reported.

"That came from other parties. We wanted to wait until we had the full understanding. I am quite confident that given our analysis that it is not more than 87 million. It very well could be less, but we wanted to put out the maximum we felt that it could be as that analysis says," Zuckerberg told media persons.

Nearly one million British users were also affected by the Cambridge Analytica data leak.

On a question if he would testify in front of Parliamentarians in the House of Commons in the UK, the Facebook CEO said: "We announced today that I'm going to be testifying in front of Congress. I imagine that is going to cover a lot of ground.

"I am going to be sending one of our top folks. I believe it's going to be Schrep, the CTO, or Chris Cox, the product officer. These are the top folks who I run the company with - to answer additional questions from countries and other places."

Zuckerberg also announced an internal audit had uncovered a fresh problem.

Malicious actors had been abusing a feature that let users search for one another by typing in email addresses or phone numbers into Facebook's search box.

As a result, many people's public profile information had been "scraped" and matched to the contact details, which had been obtained from elsewhere. Facebook has now blocked the facility.

"We wanted to shut that down because we felt like there were too many apps and too many folks who would have had access to people's content, and that would have been problematic," Zuckerberg said.

According to him, Facebook has a big responsibility when it comes to tackle fake news on its platform this year as several countries are facing general elections.

"This year is going to be an important year for protecting election integrity around the world. There's the Mexican presidential election, there are big elections in India and Brazil, as well as Pakistan and Hungary and a number of other countries, and the US midterms, of course, too," he told the media persons.

"Let me talk about how we're fighting fake news across the board. The three basic categories, are: economic actors - basically spammers - the second are governments, trying to interfere in elections - that's a security issue - the third is just polarisation and some kind of lack of truthfulness.

"So, those three streams, I think that if we can do a good job on each of those, we'll make a big dent in the problem," he explained.

Facebook now has about 15,000 people working on security and content review, and "we'll have more than 20,000 by the end of this year", Zuckerberg said.

"With respect to getting our systems under control, a couple of weeks ago I announced that we were going to do a full investigation of every app that had a large amount of people's data before we locked down the platform, and that we'd make further changes to restrict the data access that developers could get," he added.

Facebook has faced intense criticism after it emerged that it had known for years that Cambridge Analytica had collected data from millions of its users, but had relied on the London-based firm to self-certify that it had deleted the information.

Cambridge Analytica said it had bought the information from the creator of the "This Is Your Digital Life" app without knowing that it had been obtained improperly.