Social media is a new form of "word-of-mouth" as it were. It can complement your website as it gives you a voice and a way to communicate with peers, existing customers and potential new consumers. It personalizes your "brand" and helps you to spread your message in a relaxed and conversational way, and it lets you add your personality to the conversation.
Popular Social Media platforms such as Facebook, Twitter and LinkedIn are increasingly being used by organisations for interacting with consumers (B2C) as well as professionals and other businesses (B2B).
It should be noted though, that social media marketing is not the be-all and end-all of successful online marketing, but rather works better when used as the social engagement component of your overall marketing strategy.