Definitely never hire the "social media guru". This is why I said that it's critical for this person to understand business fundamentals. Social media doesn't just exist in a vacuum.
Re: trackability... There are certain ways that ROI can be measured, and should be measured. We are actually doing a webinar on this shortly. However, you also need to invest in the "unglamorous", day-to-day social media stuff, like answering customers' questions on Twitter, etc. In this use case, I liken it to email, phone, instant chat, or however else you serve your customers and constituents. They don't necessarily have an ROI, just a communication method. But you can and should track improvements in speed of response, % of queries answered, customer satisfaction, etc. I think smart companies area already tracking their social media, or at least investing time to learn; I work in social media monitoring and measurement, so I can definitely attest to that.