Facebook is facing a class action lawsuit over allegations that it monitors users' private messages. The lawsuit claims that when users share a link to another website via a private message, Facebook scans it to profile the sender's web activity. It alleges that Facebook systematically intercepts messages to mine user data and profits by sharing it with data aggregators, advertisers and marketers. Facebook said the allegations were "without merit". "We will defend ourselves vigorously," the world's biggest social networking site added. The lawsuit is claiming the greater of either $100 (£61) a day for each day of alleged violations or $10,000, for each user.