Facebook has become entangled in a dispute over advertising practices after some of its users claimed they received un-authorised charges to their credit cards while playing games on the social networking site.

A class-action lawsuit filed last week accuses Facebook, the world's fourth-largest website, and Zynga, the largest social gaming company, of illegally charging users millions of dollars by getting them to sign up for recurring payments without their knowledge.

The Facebook dispute comes as US lawmakers are looking at other internet marketing practices that can result in un-expected charges for consumers. A technique known as “post-transaction marketing” – in which customers unwittingly sign up for offers that result in recurring credit card charges – is being investigated by a US Senate committee.