When we go to any mall, we see number of offers on many products. For attraction purpose, companies offer free stuff behind almost every next product we pick. And human psychology says we all love free stuffs, doesn’t matter whether it’s useful for us or not. Same principle also applies on online word.
While using online services like social networking sites, people usually hesitate to pay money to use any particular service unless it’s very important for them. Number of peoples who generally prefers free stuffs are really huge and to attract this crowd to use any particular service, companies tries to keep it free for their users. As an alternative for this, they choose to place ads on their online sites and generate the required revenue.