Don't believe what? The post you're replying to says:
Google's main business is advertising. If you search, you are the product, not the customer.
That's just a simple fact.
Yes, they work hard to give you the best search results, and don't take payment for top listings. That's because if their search is sucky or biased, you won't use it. Just like if your local radio station sucks, you won't listen. But in both cases, delivering a good product is a means to an end: namely, selling advertising.
Do you pay Google for search? For Gmail? For any other service they offer? No? Then who does? The answer is: advertisers. Advertisers pay Google because they know you'll be using Google's services and seeing their ads. You don't pay for the "product" you get (search), so you're not really a customer. Advertisers DO pay Google for the "product" they get (your attention), so they are Google's customers.
That doesn't make them evil, any more than it makes newspapers or magazines or radio stations evil. But it is the truth, and we should keep it in mind. You can't understand a person or company unless you know their motivations.