Facebook is being sued over claims it has been scanning users' private messages for personal information to sell to advertisers.
The social networking site, is accused of monitoring messages and website links sent between users so they can profile what people read online.
This in turn allows the website, which has 24million UK members, to charge more for the information.
It is thought the information is particularly valuable because people are more likely to reveal their true interests in messages they think are private.
The alleged breach of privacy has been exposed by internet experts, who monitored whether messages that Facebook insists are private were actually being monitored.
The claim relates to messages sent directly to a friend's inbox, rather than posted on a profile wall, which can be seen publicly,
Many people use private messages to send friends links to other websites, such as news articles, shopping sites or sports blogs.
Facebook apparently tracked such messages so they could collect data on the interests of people who use the site. Advertising agencies and marketing companies are then allegedly sold this information so they can build up profiles of a person's interests, and target them accordingly.
If the linked webpage contains a 'like' button, Facebook will activate this so any company who has been 'liked' will see the person is interested in their products.
Google, Yahoo! and LinkedIn are among six companies facing accusations of intercepting communications for profit. The allegations have been made in a US lawsuit taken out by, Matthew Campbell, from Arkansas, and Michael Hurley, from Oregon.
They are seeking compensation of either $100 (£60) for each day the practice has gone on, or $10,000 (£6,000) for each of the US users who have had messages intercepted.
They claim Facebook's breach of privacy violates the Electronic Communications Privacy Act and California privacy and unfair competition laws. While the case only covers users in America, it is believed Facebook members all over the world will have been affected.
Swiss information security firm High-Tech Bridge researched their claims by sending links through the private messaging services of 50 social media sites, and experts checked to see which of these were then clicked on by the sites - showing they had been tracked.
But computer expert Graham Cluley said: 'I don't see anything necessarily wrong in principle with online services automatically scanning messages between individuals, and examining the links that they are sharing.
'If Facebook's security team didn't have such systems in place I would believe them to be disturbingly lax in their duty of care for users.
A Facebook spokesman added: 'We believe the allegations are without merit and we will defend ourselves vigorously.'
The site has previously paid $20million (£12million) to members who claimed in 2011 that it had used their data without their consent.
- Daily Mail