In terms of the potential value of the customer base, I think that’s where the big challenge lies. Social media isn’t always about directly selling product. Much of its value lies in the relationships built, the insights learned, the rumors debunked and the trust gained. How do you put a dollar value on that?

It’s a challenge because folks want to assign ROI dollar projections/results to all activities. But social media can have the same challenges in providing exact $ numbers on contributions to sales that traditional media relations has for PR pros.

I don’t know if 15% is the magic number. But I would think you could make some reasonable decisions by calculating how large that 15% is (is it 1,500? 15,000? 15 million?), what the actual potential value of that customer base is and how much you’re going to spend on social media. Do that, and you can see how what you’re spending per prospect compares to what you spend on your other marketing efforts. If it’s less per prospect, and if you think social media is going to be reasonably efficient at converting prospects to customers (this efficiency will vary depending on your industry and positioning within that industry), then you can probably make a good decision on whether to use social media.

At least that’s how I would approach it.