PALO ALTO — Plaintiffs attorneys at Lieff Cabraser Heimann & Bernstein filed a privacy class action against Facebook on Monday accusing the social networking site of scanning the content of messages it touts as private and using the information to sell targeted ads.

The 36-page complaint cites studies by security researchers indicating that Facebook systematically intercepts private messages to examine embedded URLs and assign “likes” to them as part of a business model based on aggregating user information. Though this practice is acknowledged in Facebook guidance for developers, it is not disclosed in the company’s user agreements, according to the suit filed in U.S. District Court for the Northern District of California.

“Facebook’s desire to harness the myriad data points of its users has led to overreach and intrusion on the part of the company as it mines its account holders’ private communications for monetary gain,” the complaint states.

Plaintiffs lawyers led by Lieff Cabraser partner Michael Sobol contend Facebook’s scanning of messages without users’ consent violates the Electronic Communications Privacy Act of 1986 and California’s Invasion of Privacy Act as well as the state’s Unfair Competition Law. They seek to represent a class of Facebook account holders who used its private messaging service and included URLs within the last two years.

The suit points to Facebook’s representations that its direct messaging function can be used to “privately” share information. “Facebook telegraphs through the use of the words ‘privately’ and ‘private’ that when a user sends a private message to another party, only the user and the intended recipient will be privy to the contents,” the suit states.

Sobol did not respond to a request for comment.

A Facebook spokesman wrote in an email that the company “believe[s] the allegations are without merit and we will continue to defend ourselves vigorously.”

The ability to gain insight into user behavior and build sophisticated profiles is incredibly valuable for companies like Facebook. It boasts 90 percent accuracy when it comes to reaching specific audiences, and sales of targeted advertising accounted for 85 percent of the company’s 2011 revenue, the complaint notes.

The suit also alleges that Facebook uses software and manual screening techniques to look through private messages in search of criminal activity, a practice that could also dig up incidental user information.

The action is just the latest to confront companies over how they tap user data to sell targeted advertising tools. Google and Yahoo face similar class actions in the Northern District over information gathered through their email services. In September U.S. District Judge Lucy Koh allowed the Gmail case to move forward, siding with plaintiffs that Google’s practice of scanning emails to sell targeted ads might be found to violate state and federal privacy laws.

A law firm has not yet entered an appearance for Facebook. In other privacy suits, including one targeting its “Sponsored Stories” feature, Facebook has turned to Cooley’s Michael Rhodes.

Contact the reporter at callison@alm.com.