(Adds committee briefings, advertising comment, updates shares)
WASHINGTON/LONDON March 22 (Reuters) - Facebook Inc came under further pressure from lawmakers, investors, advertisers and users on Thursday, the day after Chief Executive Mark Zuckerberg admitted the social media network made mistakes in letting 50 million users' data get into the hands of political consultancy Cambridge Analytica.
U.S. lawmakers demanded Zuckerberg personally testify in Washington to explain his company's actions.
Meanwhile, advertisers Mozilla and Commerzbank suspended ads on the service and the hashtag #DeleteFacebook remained popular online, although it was hard to tell how many users are abandoning Facebook.
In light of those concerns, investors continued to sell off Facebook shares.
The company has lost more than $50 billion in market value since allegations this week that political consultancy Cambridge Analytica improperly accessed data to build profiles on American voters and influence the 2016 presidential election. Five days after the scandal broke, Zuckerberg apologized on Wednesday for mistakes his company made and promised to restrict developers' access to user information as part of a plan to improve privacy protection.
On Thursday, Facebook executives were still saying sorry. It was a mistake, Campbell Brown, head of news partnerships at Facebook, said at The Financial Times FT Future of News Conference in New York City.
Zuckerberg's apology and promises were not enough to ease political pressure on the world's largest social media company.
"It shouldn't be for a company to decide what is the appropriate balance between privacy and innovation and use of data. Those rules should be set by society as a whole and so by parliament," British minister for Digital, Culture, Media and Sport, Matt Hancock, told BBC Radio.
Zuckerberg's media rounds did little to satisfy Washington lawmakers in either political party who have demanded this week that the billionaire testify before Congress.
The Republican chairman and top Democrat of the U.S. House Energy and Commerce Committee said they will in coming days formally ask Zuckerberg to testify.
The request came a day after company executives briefed committee aides for nearly two hours and left with a list of at least 60 questions they were unable to answer, according to two aides.
Zuckerberg said in media interviews on Thursday he would be willing to testify if he is the right person at the company to speak to lawmakers.
My message to Mark Zuckerberg is you are right person. Theres no question you are the right person, Senator Richard Blumenthal, a Democrat from Connecticut, told reporters after Facebook executives briefed Senate staff on Thursday.
Facebook said it had no further comment beyond what Zuckerberg had already said.
Facebook executives also briefed another House committee on Thursday, and while they answered many questions about Cambridge Analytica they were unprepared to address broader questions about Facebooks data privacy policies, according to a staffer who was present
Wall Street analysts expressed relief that there were no signs so far of a more fundamental shift in the company's advertising-driven revenue model, but some said there would be costs to shore up its reputation.
Facebook, with more than 2 billion monthly active users, made almost all its $40.6 billion in revenue last year from advertising.
Several analysts cut price targets and Facebook shares were down 2.2 percent on Thursday in heavy trading. Technology stocks have fallen along with Facebook this week as investors worried about tighter scrutiny of global platforms like Alphabet's Google, Twitter and Snapchat.
Analysts said that Zuckerberg's promises to investigate thousands of apps, and to give members a tool that lets them turn off access, would not substantially reduce advertisers' ability to use Facebook data - the company's lifeblood.
Nevertheless, open-source browser and app developer Mozilla said it was "pressing pause" on its Facebook advertising after the revelations prompted it to take a closer look at the site's default privacy settings.
"We found that its current default settings leave access open to a lot of data particularly with respect to settings for third-party apps," Mozilla said in a blog post.
Commerzbank paused its campaign on Facebook. "Brand safety and data security are very important to us," head of brand strategy Uwe Hellmann told newspaper Handelsblatt. The comments were confirmed by a spokesman for the bank.
British advertising group ISBA, which represents thousands of brands, threatened to withdraw ads if investigations show user data has been misused.
"We think this issue is more likely to snowball than recede and that advertisers are reaching a tipping point at which spending on not only Facebook and other online platforms, is re-evaluated," brokerage Liberum said in a note.
But Sharon Rowlands, president of USA Today Network Marketing Solutions, said it was unlikely a flood of brands would stop advertising on Facebook because it is good at targeting their customers and generating a return on investment.
"The challenge is that the platform performs," Rowlands said.
(Additional reporting by Dustin Volz and David Shepardson in Washington, Jessica Toonkel in New York, Munsif Vengattil in Bangalore and Paul Sandle in London Writing by Susan Thomas, Editing by Nick Zieminski)