Clicked on Linkedin and it came back with, "This question is no longer available." Thank god it didns't say its no longer relevant, hehe.
But on to your question, would I hire a social media strategist? Yes I would if my company's success is highly predicated on online crowdsourcing. Just because its "bubblish" doesn't mean there's no money to be made. In this I take the low road. In fact I think everything is a bubble - either overvalued or will some day peak and decline in value so I don't see bubbles as necessarily bad. The real issue is how to take advantage of transient bubbles and make a profitable exit. If there's a lesson to learn from the dot com bust, its the heightened respect for exit plans. The social media strategist will be the sentinel who sounds the alarm when the tide recedes.
Qualifications? A campaign strategist with demonstrable experience in social media. I treat all commercial ventures (including product marketing) as a collection of campaigns with measurable results. As for traditional media, I'll outsource.
On returns I'll take the simple path: campaign revenue per headcount.
You've posed an excellent question. I'd be interested to read the findings to your Linkedin's post if you would share them.