Facebook and Cambridge Analytica are facing multiple lawsuits over alleged misuse of personal information as the social media giant began notifying millions of users their data may have been harvested without their knowledge.
At least five law firms in the UK and US are investigating claims for compensation after thousands used an app which collected data about users and their Facebook friends.
Facebook on Tuesday began notifying 87 million people worldwide, including nearly 1.1 million Britons, that their personal information on the platform may have been given to Cambridge Analytica (CA), a UK consultancy accused of using personal data to help politicians target advertising around elections.
Facebook insists it has not broken any laws, but the tech giant's founder and chief executive Mark Zuckerberg admitted last week that the company "didn't do enough" to prevent firms using his platform "for harm".
Mr Zuckerberg is due to answer questions from US Congress on Tuesday and Wednesday over the scandal of how users' data may have been used by CA to help Donald Trump's election campaign target voters.
Ravi Naik, from ITN Solicitors in London which is pursuing legal action on behalf of clients, said: "Facebook has taken the first step to accountability, which is accepting there is a problem. But they need to explain what they are going to do to compensate people for the breaches and remedy the problem."
Lawyers from ITN Solicitors, Leigh Day and McCue & Partners in the UK, and RuyakCherian, Fields and Cross & Simon in the US are all pursuing legal action on behalf of clients.
Mr Naik said his firm will be seeking clarification about how Facebook knows which users were affected by the breach and exactly what information was transferred.
A number of people posted screengrabs of messages they had received from the social network on Tuesday, saying either that they were unaffected, or that their information may have been harvested as a result of them or one or more of their friends using the quiz app This Is Your Digital Life, from which the information was collected.
Matt Rose, who runs a marketing and content agency in London, said he did not receive a notification from Facebook, as the firm had promised, and only found out his personal information had ended up in the hands of CA after looking for it himself.
He told the Press Association: "I think we are all used to knowing that the actions we take online are monitored, but not that the actions other people take can have a direct effect on us and our data, whether we like it or not."
In telling users if they were affected, Facebook says the "public profile, Page likes, birthday and current city" was "likely shared" through the app, but added: "A small number of people who logged into This Is Your Digital Life also shared their own News Feed, timeline, posts and messages which may have included posts and messages from you. They may also have shared your hometown."
Facebook stopped allowing app developers to collect information from users' friends in 2014. It declined to comment on the proposed legal action.