A French consumer rights body on Thursday warned Facebook, Twitter and Google+ of legal action if they do not bring privacy procedures into line with French law on data protection within three weeks.
The threat from UFC-Que Choisir comes after France's national data protection agency Cnil said last week it could impose a fine of up to 150,000 euros ($198,000) on Google if it did not change its privacy practices, including clarifying its intentions and methods for data collection.
UFC-Que Choisir president Alain Bazot said the three sites had been informed of the deadline on Wednesday, adding that their current privacy procedures "makes one shudder".
"Data is collected in a dubious manner from all sides and used infinitely," Bazot said in a telephone interview. "If there is no response in 21 days, UFC reserves the right to take legal action."
He said the sites' terms of service were opaque and vague, adding that the texts "only available on their Internet sites, are unreadable."
Amal Taleb from the UFC's legal department said the booming use of social networking sites made the problem more serious. He said 90 percent of the Internet users in France were members of a social networking site.
"There are great worries about this," he said.
In 2012, Google launched a new privacy policy enabling it to track users' activity across its search engine, email service, the Google+ social networking platform and other services it owns, which include YouTube.
The changes make it easier for Google to collect and process data that could be used by advertisers to target individuals with offers tailored to their specific interest, thereby increasing the company's revenue potential.
The changes have been widely criticised because of the implications for privacy but the pressure on Google to change how it operates has been limited.
The 27-member European Union warned Google in October 2012 that its data protection procedures did not comply with an EU directive on the subject and gave the company four months to change them.
That deadline passed without any action, prompting France to set up a task force of individual member states interested in pursuing the issue that involved Britain, Germany, Italy, the Netherlands and Spain.