For the second time this month, Facebook has been sued by an advertiser for click fraud. This latest case, filed Monday by software marketer United ECM, alleges that Facebook charged marketers for "non-existent, fraudulent or invalid clicks." United ECM, which is seeking class-action status, brought suit in federal district court in San Jose.

"Based on plaintiff's records, Facebook has overcharged plaintiff," United alleges in its complaint.

United does not provide other details about the size of the discrepancy or how long it advertised on Facebook. The complaint extensively references a June 21 TechCrunch post about alleged click fraud on Facebook, which also appeared on The Washington Post's Web site.

That item discussed recent marketer complaints made on the WickedFire forum. At the time, Facebook acknowledged that it had seen an increase in "suspicious clicks" and was rolling out a fix. The company also said it was identifying advertisers who had been affected and would credit their accounts.

Earlier this month, sports site RootZoo sued Facebook for click fraud based on allegations that the site charged for clicks that never occurred. RootZoo, which advertised on Facebook from November of 2007 through June of 2008, said its found discrepancies between data provided by its own analytics programs and Facebook's numbers. RootZoo alleged that on one day in June, its software programs showed that 300 clicks had been generated by Facebook, but the company was charged for 804 clicks.

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