Websites sell a lot of ad space, but who’s looking at those advertisements? Well, the good news is that most of them are human. Most. A recent analysis shows that about 36% of Web pageviews are fake, generated by shady entities in Eastern Europe, not humans who might be interested in buying cars or makeup. The big companies that make the largest ad buys are not pleased.

As spending on ads served up on both the desktop-based Interweb and mobile apps increases, advertisers have a dilemma: their customers are online, and they can’t afford not to advertise online. At the same time, they’re afraid that they might be paying for ad views on zombified PCs, not real people with real wallets.

How much money are we talking about? The Wall Street Journal estimates that advertisers in the U.S. will spend about $50 billion to beam their various messages out to you this year. Meanwhile, one estimate is that advertisers paid for $5 billion worth of fraudulent traffic last year.

The solution for advertisers isn’t to give up on advertising online, but to watch more carefully for fraud. Big companies like GM and Lenovo are hiring their own auditors to make sure their ads reach real people.

A ‘Crisis’ in Online Ads: One-Third of Traffic Is Bogus [Wall Street Journal]