Facebook, the world's largest social-networking Web site, was sued over claims it charges advertisers more than it should, based on the number of users viewing the ads.

The lawsuit, filed today 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 suit. 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 Inc. seeks class action, or group, status for the suit, arguing it was "wrongfully charged for non-existent, fraudulent or invalid clicks."

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

Facebook spokesman Barry Schnitt didn't immediately return a call seeking comment.

The case is Unified ECM v. Facebook, U.S. District Court for the Northern District of California (San Jose).