Facebook is planning to move its UK users on to US agreements, where data privacy laws are less stringent than the European Union regulations currently in force.
The move is the legal ripple effect of Brexit, when the UK will no longer be treaty-bound to EU laws.
The change will happen next year and follows a similar move announced in February by Google.
"Like other companies, Facebook has had to make changes to respond to Brexit and will be transferring legal responsibilities and obligations for UK users from Facebook Ireland to Facebook Inc," the company's UK arm said.
"There will be no change to the privacy controls or the services Facebook offers to people in the UK."
Immediately after Brexit, the UK will carry on tracking the EU's General Data Protection Regulation, one of the most stringent data-protection laws in the world, but it is expected, over time, to diverge.
Data privacy activists in the UK fear that Facebook may pressure the US government into forcing Britain to accept US data laws in any future trade deal which could be negotiated after Brexit.
Some also worry that UK Facebook users could more easily be subject to surveillance by US intelligence agencies or data requests from law enforcement.
"The bigger the company, the more personal data they hold, the more they are likely to be subject to surveillance duties or requirements to hand over data to the US government," said Jim Killock, executive director of the UK organisation, Open Rights Group.
US courts hold that constitutional protections against unreasonable searches do not apply to non-citizens overseas.
Information industry regulators in the UK said they had been in touch with Facebook and other tech companies with EU bases before Brexit.
“We are aware of Facebook’s plans and will continue to engage with the company in the new year,” said a spokeswoman at the Information Commissioner’s Office.
A Twitter spokesman said its UK users will continue to be handled by the company's Dublin office.
Facebook will inform users of the shift in the next six months, a company spokesman said, giving them the option to stop using the world’s largest social network and its Instagram and WhatsApp services.
Facebook's decision comes at a time when the UK is escalating efforts to ban strong encryption, which Facebook is moving to implement on all its products.
The UK, like the EU, is also pressuring Facebook on a number of other fronts, including hate speech and terrorism policies.
The US may also pursue new laws on privacy and social media content, and federal and state prosecutors recently launched antitrust lawsuits against Facebook and Alphabet, Google's parent company.
