You cannot call google a monopoly because there are other search engines that exist in the free market. In the free market, buyers are free to advertise at the search engine of their choosing. Google is not going to show advertisement for ads placed in yahoo or bing.

By definition, monopoly exists when a specific person or enterprise is the only supplier of a particular commodity. In Google's case it is not. It's just like a different brand of rice that everyone prefers.