Your competitors are already there – Whatever industry you're in, I can guarantee you that your competition is already on social media, and is reaping the rewards as a result.
It's the best way to connect with your customers – Customers are online more than ever. If they have a question, a query, or just want to show their high regard for you business by liking your Facebook page, you should give them the chance to do so.
You can respond to problems quickly – As soon as an issue is flagged by a complaint on Twitter, Facebook or any other social media platform, you can jump on it and respond online. Better to deal with issues that customers bring up, than have them badmouthing your company online somewhere else.