Social media provides a way for businesses to increase their "word of mouth" advertising. Word of mouth has always been a powerful method of promoting products and services.
Social networking makes it possible to establish relationships and build trust online, which allows businesses to indirectly market their products and services to people who are more receptive to the marketing messages you are sending out. Generally, people are more trusting of social media sites than they are of more traditional websites selling products or services directly, because the users of social media websites feel that there are fewer people who would have something to gain by reviewing and recommending another's product or service.