I have been encouraging companies who are getting into social media to get a community manager, and even explaining the concept of the Community Manager, many people still do not get it. The person they tend to hire is a Social Media Manager, and there is a difference.
Social media manager
A social media manager takes the lead in managing the overall efforts of a company social media platform. He is responsible for creating content specific to a product or brand strategy across social networks. They monitor and create engagement to achieve goals of a specific campaign, and measure ROI.
A community manager's role is mainly to monitor and engage the public for that community. After which, the community manager will feedback what is learned from the community to the company. I consider a community manager to be more detailed and involved in certain aspects of social media, whereas a social media manager can manage more of the bigger picture aspects of online marketing.
Do you believe they vary for a small business vs. an enterprise?
Usually, an enterprise would hire several social media managers, and they may not be even in charge of developing the strategy, however for small businesses, a social media manager would wear many hats and focus on one or 2 channels.
A community manager in a small business may actually take on the role of social media manager, having many responsibilities including creating content, however a community manager in a large enterprise may just be in charge of engagement on one or 2 channels.
With a good community manager and social media manager, a company or an enterprise can monitor brand and prevent social media crisis from happening.
-- Robin Low