The suit cites independent research that it claims has found that when a user shares a link to another website in a private message, it is recorded to contribute to a profile of the sender's web activity.
It also said the guidance Facebook gives to web developers states that a link in a private message can contribute to the number of "likes" that a page – which can represent, for example, a company or a band – receives.
The class action has been brought by Facebook users Matthew Campbell from Arkansas and Michael Hurley from Oregon, on behalf of all Facebook users in the U.S. who have sent links via private messages. It said the number is likely to be in the millions as there are more than 166 million Facebook account holders in the country.
(Read more: Facebook monthly active users)
"Representing to users that the content of Facebook messages is "private" creates an especially profitable opportunity for Facebook, because users who believe they are communicating on a service free from surveillance are likely to reveal facts about themselves that they would not reveal had they known the content was being monitored," said the suit, filed this week with the U.S. district court for Northern California.