SAN FRANCISCO (Bloomberg) - Facebook, the world's largest social-networking website, was sued over claims it charges advertisers more than it should, based on the number of users viewing or clicking on their Internet ads.

The lawsuit, filed yesterday in federal court in San Jose, California, was brought by Beverly Hills, California-based Unified ECM Inc. Companies that advertise on Facebook have two options, according to the complaint. They must pay a specified amount each time a user clicks on an ad, or pay an amount for each 1,000 page views, or impressions, of advertisements.

"Facebook represents that the amount the advertiser is charged will be based on the number of clicks it receives or impressions that are viewed," according to the complaint. Unified ECM seeks class, or group, status for the suit, arguing it was "wrongfully charged for non-existent, fraudulent or invalid clicks".

The lawsuit claims Facebook is not doing enough to monitor or prevent "click fraud", which occurs when a person or company with no intent to shop clicks on an ad to hurt a competitor by inflating the number of visits on which bills are based. In 2006, Google Inc., the most-used Internet search engine, paid $90 million to settle click-fraud claims.

When customers suffer from click fraud, Facebook "fails to adequately advise them that they have become victimized, and fails to refund them the full excess charges that they have incurred as a result of the fraudulent click activity", according to the complaint.

Facebook, based in Palo Alto, California, has about 250 million users, who post updates about their daily lives and share pictures with friends.

Barry Schnitt, a spokesman for Facebook, said he has not seen the lawsuit and declined to comment immediately.