First of all, you need someone who thinks strategically and will do the research to plan just how, why and where you want to engage in social media. That's where marketing comes in. New media is needed to make sure your branding is executed and your digital assets are properly formatted for each social network, virtual world or blog that you're leveraging. PR and communications skills are imperative to understand how to pitch and interact with the public.

Of course these skills need to be augmented by experience using social networks, and a deep understanding of how they work, how users interact, and to know how to show restraint in your engagement there.

To the last point, I find it hard to believe that too many companies have the in-house staff to dedicate to their social media efforts. A social media engagement is not something you spend a month doing. It may take 3 months to get ramped up, then should be continuously managed and tweaked. Like email and PPC campaigns, social media can be tweaked on an hourly and daily basis. The difference is, that with SoMe, you need to be timely with responses or you will be ignored, even hated. Like email and PPC, even web design strategies, you also need someone who knows the right tools and how to use them to measure your efforts.

The upshot of these last two points is that managing social media engagement is a full-time job with a highly unique and versatile skill set.