A 'Crisis' in Online Ads: One-Third of Traffic Is Bogus As Digital Advertising Climbs Toward $50 Billion This Year, Marketers Battle Fraudulent Visitors Billions of dollars are flowing into online advertising. But marketers also are confronting an uncomfortable reality: rampant fraud.   About 36% of all Web traffic is considered fake, the product of computers hijacked by viruses and programmed to visit sites, according to estimates cited recently by the Interactive Advertising Bureau trade group.   So-called bot traffic cheats…