Millions of Facebook users have had their personal information leaked to advertising companies through various Zynga games, it has emerged.
A wall Street Journal investigation into the matter concludes that all of the 10 most popular apps on Facebook had been transmitting user IDs to external companies, igniting new criticisms of privacy defences on the social network.
The probe implicates six Zynga games: FarmVille, Texas HoldEm, Café World, Mafia Wars, Treasure Isle and FrontierVille.
The Wall Street Journal adds: “three of the top 10 apps, including FarmVille, also have been transmitting personal information about a user's friends to outside companies.”
At least 25 advertising and data firms have been collecting the Facebook user ID, the report found, with several of those building user profiles, and – in some cases – selling them on to other companies.
One company, RapLeaf, was said to be selling its information. The firm sent its data to at least twelve advertising and data firms – including Google Inc.'s Invite Media. The twelve companies have denied collecting, storing or using the information.
A Facebook spokesperson said the group will seek to "dramatically limit" the exposure of users' personal information.
A statement read: "A Facebook user ID may be inadvertently shared by a user's Internet browser or by an application”.
The group insisted that the IDs shared to third parties will not allow “access to anyone's private information.
It isn’t known how long users’ IDs have been traded, but it appears that the data has been transmitted without Zynga’s knowledge.
Facebook has temporarily shut down some offending apps – though not Zynga’s – and said will build new technology to contain the problem.
A Zynga spokeswoman said, "Zynga has a strict policy of not passing personally identifiable information to any third parties. We look forward to working with Facebook to refine how web technologies work to keep people in control of their information."
A Facebook ID is a number assigned to every user on the site, which is publicly available.
Games on Facebook have become a huge business. The social network says 70 per cent of its patrons use apps each month.