A public apology by Facebook chief Mark Zuckerberg failed to quell outrage over the hijacking of personal data from millions of people, as the tech giant ended the week $58bn lower in value in the aftermath of the scandal.

Cambridge Analytica, which worked on US President Donald Trump's election campaign, has been accused of illegally mining 50 million Facebook users' data and using it to target potential voters.

The row has sparked a major crisis for Facebook, prompting investigations on both sides of the Atlantic and sending its share price plunging amid fears of additional regulation that could affect its business model.

Zuckerberg on Wednesday, five days after the news broke out, issued the public apology, admitting there had been a "major breach of trust".

But the apology did not stop investors from selling shares in Facebook, with many wondering just how bad the damage would be for the social network.

The breach was called a "light bulb" moment for users, spawning the social media trend #deletefacebook.

With all the negative headlines, shares in the social media company fell from $176.80 on Monday to around $159.30 by Friday night.

Hargreaves Lansdown senior analyst Laith Khalaf said the week had been a "damaging episode" in Facebook's history.

"One of the secrets of Facebook's success has been that the more people who use Facebook, the more integral it becomes to its customers. Unfortunately for Facebook, the same dynamic cuts in the opposite direction if it loses a meaningful number of users as a result of this scandal. "

Advertising firm M&C Saatchi's founding director David Kershaw said the revelations that a 2014 Facebook quiz essentially harvested data from users and their connected friends without consent have led to a backlash from advertisers.

"Clients have come to the point, quite rightly, where enough is enough, " Kershaw said.

Advertisers Mozilla and Commerzbank on Wednesday suspended ads on the social media platform.

On Friday tech entrepreneur Elon Musk had the official Facebook pages for his companies Telsa and Space X deleted, reported Reuters.

"Make no mistake Facebook is an amazing medium from the advertiser's point of view because of the accuracy of its targeting - which comes from data. But I think those large companies are very nervous to be associated with a medium where the data is being abused, particularly in a political context," Kershaw said.

Kershaw told the BBC any change in Facebook's data protection policy was more likely to come from the threat of a withdrawal of "hard money from advertisers rather than consumers running hashtag [campaigns] on Twitter," referring to the #DeleteFacebook and #BoycottFacebook hashtags that have become popular.

It will take some time before it becomes clear if the advertising industry's dissatisfaction with Facebook leads to them actually pulling their money out of the social network, or whether the howls of condemnation amount to mere posturing from a group of concerned clients.

Fresh allegations also emerged Friday night about Cambridge Analytica's involvement in the 2016 Brexit referendum campaign, reported AFP.

Brittany Kaiser, CA's business development director until two weeks ago, revealed it conducted data research for Leave.EU, one of the leading campaign groups, via the UK Independence Party (UKIP), according to The Guardian.

Technology writer Kate Bevan said the week's events have woken Facebook's users up to the fact that the platform's games, quizzes and apps could harvest their data for more serious intents.

"This week feels to me like a real light bulb moment where people are understanding that it's not just clicking 'like' on Facebook, it's giving your data away".

The sentiment was echoed by the European Union's commissioner for justice, consumers and gender equality Vera Jourova who said the Cambridge Analytica allegations had been "a huge wake-up call" for Facebook users about the demand for their data.

"The tiger has gotten out of the cage".