The French government on Tuesday summoned Facebook managers to explain rumours that some users' privacy had been breached with private messages posted publicly on the social network.
Facebook denied the rumours that personal messages were appearing on the users' "Timelines" which can be accessed by a large Internet audience.
But a French minister said doubt remained and urged users to file complaints if they felt their privacy had been violated.
In a joint statement, Industrial Renewal Minister Arnaud Montebourg and the junior minister for the digital economy, Fleur Pellerin, said Facebook managers had been summoned before France's CNIL data watchdog to explain the rumours.
"Clear and transparent explanations must be given without delay," they said.
"This incident underlines once again the importance of protecting personal data in the digital world and the lack of transparency in handling them."
A meeting between Facebook managers and CNIL officials began at lunch-time and was still underway, CNIL head Isabelle Falque-Pierrotin said.
Facebook on Monday said it had investigated complaints from members and denied the reports of private messages being made public.
"A small number of users raised concerns after what they mistakenly believed to be private messages appeared on their Timeline," the California-based social network said in an email response to an AFP inquiry.
"Our engineers investigated these reports and found that the messages were older wall posts that had always been visible on the users' profile pages."
Concerns that private Facebook messages from 2007, 2008, or 2009 were being posted for public viewing spread wildly on Twitter Monday after a story first appeared in the free French daily Metro.
"Facebook management is unable to give us any explanation of what happened," Pellerin said on i-Tele television. "Today complete confusion reigns and Facebook's explanations are not very convincing."
"If there is ever any real certainty that private messages were made public and that there was a breach of confidentiality... I would advise people to file a complaint. This is unacceptable," she said.
A Facebook user said two private messages had cropped up on her Timeline among many other public ones wishing her a happy birthday.
"One of them was from a colleague on maternity leave ... and the other was from a friend, a very private message about her boyfriend who had ditched her," she told AFP.
Facebook has about 26 million users in France.
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