While most of Facebook's employees are celebrating today's initial public offering, at least a few members of its legal department are likely hard at work preparing to fight a $15 billion class action lawsuit filed on Friday in connection with the social network's tracking of users' online activity.

The lawsuit — filed in a San Jose, Calif. federal court — combines more than 20 similar cases from around the United States, according to Bloomberg.

The suit's organizers are accusing Facebook of invading users' privacy by tracking their movements online, which the site does via "like" buttons embedded on sites across the web.

“This is not just a damages action, but a groundbreaking digital-privacy rights case that could have wide and significant legal and business implications," a partner at Stewarts Law US LLP, one of the firms leading up the suit, told Bloomberg.

Plaintiffs in the case are claiming that Facebook's user tracking violates the U.S. Wiretap Act, which prohibits the "interception and disclosure of wire, oral or electronic communications." According to the lawsuit, the Wiretap Act "provides statutory damages of the greater of $100 per violation per day, up to $10,000, per Facebook user," which amounts to $15 billion for Facebook's more than 800 million members in total.

The class action suit is the result of a decision made by a California judicial panel, which decided the multiple lawsuits should be unified and heard in Facebook's home state.

Stewarts added that the firm is trying to find ways to add international claims to the U.S. lawsuit. Facebook's privacy practices have recently come under heavy scrutiny in Europe.

Facebook's shares were set to trade on Nasdaq at $38 each on Friday morning, raising $16 billion and valuing the company at just over $104 billion total.

Mashable reached out to Facebook for comment, but has not yet received a reply.