The secretary / roi of something taken for granted argument is common and all things said, not half bad, but it presupposes that social is unmeasurable like an employee. While nailing down any ROI from social can be hard, and the full, bottom line impact is simply impossible, you can quantity parts just as you can quantify the minimum value of an employee. Thus rather than comparing two abstracts I'd rather look at social from another side: the opportunity.
First we have to define what we're trying to measure. At it's root "social" is merely a word that, while often attached to specific channels & tactics like being on facebook, describes a much larger phenomena, a shift in the way the world works. In the past the company controlled the message, the offer, the amount of information. But the internet gave people access to all information and then social interactions took that a step forward and gave them a voice to add to it. It's power unlike anything before it.
This gets us to the result: We've got a world in which people use "social" for everything. Literally. There's studies showing the impact of reviews on buying household goods that cost a few dollars. There's entire products built around viral spread. And there's ever growing expectations for response time and interaction on social named channels. The first ROI mistake is in assuming social is a marketing tactic and thus about incremental spend for incremental return. Engaging is just as much about keeping what you have going just as much as adding too it.
You can ignore it, you can dismiss it but whether you participate or not the conversation is happening and will be about you in whatever form you want. To quote a source I can't recall [think it was @unmarketing]: when the phone rings you answer it, when an email comes, you reply, why should a tweet by any different?
Finally we get to the evaluation part. No doubt social is simply essential but how much do you put into it, and with what expectation?
When I sit down with a business and they ask me what they will get for investing in social, my response usually starts with a question back: What would you pay to be able to reach your customers such that they could respond back to you? What would you pay to shift your support off the phone? What would you pay to have people sell your business for you without having to pay them?
You have to walk in with the right understanding of what "social" means and why it matters before you can measure it. That's because what you measure is limited by precisely what you are trying to do... in fact the better you are at social, the harder it is to find ROI, or even to see social stats. You can, and should, count the direct contributions, the interactions / followers [social stats], the business correlations [clicks, calls, leads] and the hard numbers [known sales]. Then you have to look at the savings in support, R&D and launches as well as the lift [often back to baseline] of validation, of having something authentic.
What you're left with isn't one number but rather a piece work of social stats and dollars which -- if you've done your job right -- are seen as an indicator of the larger contribution. Something to know your direction, to take a pulse from, but by no means the absolute bottom line.