The 10 most popular Facebook applications have been sending users' personal identifying information to advertising and internet tracking companies, according to The Wall Street Journal.
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Facebook user IDs could be included in the referrals that websites send to other sites to tell them where the user originated, the paper said.
In a statement, Facebook said the report was exaggerated and there was no evidence any personal information was misused or even collected.
But the social networking site has disabled the offending applications and undertaken to find technical solutions to the problem.