Facebook has taken action against developers it caught selling user names and contact lists.
The sales were uncovered as Facebook investigated a web browser bug that let user IDs be shared inadvertently.
The user details were sold to data brokers who used the information to target adverts more precisely.
Facebook started investigating what was happening with user identifiers (UIDs) following media reports that the information and lists of contacts were being sold on to advertising firms.
It also discovered that some developers that create applications for the social network were taking the user IDs of those who used their creations and selling them on.
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