Congratulations! You've decided you a social media manager! Now you can stop wasting your time talking to customers online, or blogging, or anything else in that weird online world. She can handle it all, right? Wrong. Social media is a tool, not a technique. Social media is another way to talk to current and potential customers. Like the phone on your desk, or e-mail. Your social media manager's job is not to take the job of online communications away from everyone else. Her job is to help everyone else use it to better help customers and build the business.
Her job is to Continuously train you and your team to talk with customers on Twitter, Facebook, via your company blog and through whatever other channels your customers use. She will Monitor conversations about your brand. Your social media manager operates your company's online listening post. Her job is to know when someone speaks well or ill of you before you do. Oh - 'monitoring' includes 'analytics', by the way.
She must craft strategy. She's the person who figures out how social media will help your company grow, and how to tap it as a resource. It's another arrow in your internet marketing quiver. Her job is to make sure that arrow lands on target.
A social media strategy, and a person to drive it, are no longer optional. Nor is getting every single customer-facing person at your company comfortable using the most common channels, like Twitter.