They claim independent research found that Facebook scans messages for links to other websites and other data, and then profits from it by sharing it with data aggregators, advertisers and marketers.
The lawsuit says Facebook users are likely to reveal facts about themselves that they would not otherwise because they believe they are communicating on a service free from surveillance.
"Thus, Facebook has positioned itself to acquire pieces of the users' profiles that are likely unavailable to other data aggregators,” the lawsuit says.
Facebook has denied the claims, saying they are "without merit,” adding that the company will defend itself “vigorously”.
More on Facebook privacy
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- Facebook to boost privacy controls
- How to reassess privacy settings in wake of Facebook cloaking issues
The lawsuit's plaintiffs hope to gain a class-action lawsuit that encompasses all Facebook users who have sent or received a private message on the social network in the past two years, reports Slashgear.
The claimants are seeking either $100 a day for each day of alleged violations or $10,000 for each user, according to the BBC.