How search engine and social network owners earn money ?

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Hello there, often I wonder how the owner of a search engine or that of social networking site earns money. For example, If I want to search something on google, I can search without paying a penny even. On the other hand google have to spend a lot of money in order to make its search engine available for its users. But just without being paid. Similarly we create our profiles on facebook, for instance, and facebook maintains our data without being paid. Same is the case with sites like yahoo and hotmail etc. I wonder who pays them if we use their services for free? Can anyone help me here please? Thanks