Since you asked, Matthew...
As a fellow 'mature marketer,' I agree with your poke at today's young brand managers jumping willy-nilly on the 'social marketing' band wagon. I disagree that they are doing it primarily because they think it's the cheapest media out there (although they're likely using that line with their bosses and CFOs!) -- their primary motive is to be seen as insightful leaders of the pack. Ironically, an 'investment' in social is going to prove them to be lemmings!
So, how to convince these 'marketing mavericks' that anything labeled social will rarely lead to sales? Very simply by pointing out the difference between totally appropriate spending on PR (an attempt to earn goodwill and win 'free' media mentions and inter-consumer buzz at low cost) and spending on marketing efforts.
Anything labelled 'social' is, and always has been, a PR medium. No more, no less. When we spend money on PR, we don't EXPECT ROI, we hope for it. Kind of like Unilever Canada's so-called 'investment' in the "Bridezilla Wigs Out" YouTube video of a few years ago -- an experiment that led to lots of buzz, but zero ROI for Sunsilk.
Today's new 'social media' are no more appropriate for carrying advertising messages (marketing efforts) than the last 'social medium' that came along, the telephone. Once marketing people (not all of whom are all that insightful...) 'get' the distinction, we'll be back to looking for the next great marketing-appropriate wave and 'social' will return to its rightful place in the PR budget.