It's been a week of ups and downs—mostly ups—for Facebook founder Mark Zuckerberg. On Friday, the social network company started publicly trading with a market close that valued the one-time startup at $104 billion. And then the very next day, Zuckerberg married his longtime girlfriend, Priscilla Chan, in a secret ceremony they told their family and friends would be a party to celebrate Chan's medical school graduation.

But only days before these monumental events, Facebook was slapped with a $15 billion lawsuit by some subscribers who claim the company violated their privacy. In a complaint filed last week, the Facebook users claim the company improperly tracked their Internet usage even after they had logged out of their accounts. Twenty-one cases with similar claims have been consolidated before a federal court in California, and the suit represents users who subscribed to Facebook between May 2010 and September 2011.

The accusations aren't new for Facebook. For years, the company has fought allegations from regulators that it plays fast and loose with its privacy policy. Late last year, it settled a suit the Federal Trade Commission had filed against it claiming similar privacy violations.

“This is not just a damages action, but a groundbreaking digital-privacy rights case that could have wide and significant legal and business implications,” David Straite, a partner at Stewarts Law, which represents some of the users, said in an emailed statement.

Facebook spokesman Andrew Noyes said in an emailed statement that the claims are meritless and the company plans to fight them.