When was the last time you bought something based on something you saw on Twitter or Facebook?

Can’t think of an example? I can’t either. I see the Facebook ads and the sponsored Tweets. But nestled in a constant stream of information, they’re just too easy to overlook or ignore. I don’t think I’m alone here and that observation probably concerns many marketing departments.

Corporations wants to jump onto the social media bandwagon. But chief marketing officers so far have yet to see much bang for their buck. Just 15 percent of CMOs report seeing proven, quantitative impact from their social media investments, according to a survey by Christine Moorman, a professor in Duke University’s Fuqua School of Business. To hammer home the point even more, nearly half of CMOs surveyed said they have not been able to show that their social media activities have made any difference.

The survey included responses from 410 CMOs. Duke has conducted this CMO survey twice a year since 2008.

Companies have shown the willingness to spend a great deal of money on advertising to support a brand or a product. If you doubt that, look at ad dollars spent for the Super Bowl. Even so, the Super Bowl comes but once-a-year event and companies are trying to reach audiences the other 364 days of the year. The shrinking of traditional advertising budgets comes as companies pour more money and resources into social feeds reaching our laptops, tablets and smartphones.

When the lights went out during this year’s Super Bowl plunging half the Superdome into darkness Oreo famously Tweeted, “Power out? No problem. You can still dunk in the dark.”

The timely Tweet went viral and helped the Oreo brand. But did it boost sales? Oreo cookies are made by Nabisco, which is a subsidiary of Mondelez International (NASDAQ:MDLZ). Mondelez’s earnings don’t crumble down to Oreo sales but for the first half of 2013, the company reported $2.6 billion in biscuit sales, up 4.1 percent compared to the same period in 2012. I can’t tell how much of that increase came from the social media efforts around Oreo and other Nabisco products. Perhaps more important, maybe Mondelez can’t tell either.

Increased social media spending without measurable results puts CMOs at risk. Of the CMOs surveyed by Duke, 66 percent reported feeling more pressure from CEOs and boards of directors to prove the value of this marketing.

“Marketing leadership requires that CMOs offer strong evidence that strategic marketing investments are paying off for their firms in the short and long run,” Moorman says in her blog post about the survey. “CMOs will only earn a ‘seat at the table’ if they can demonstrate the effect of their marketing spend.”

And yet, even without that demonstrated value, corporate social media spending is only trending upward. CMOs surveyed expect to increase social media expenditures from 6.6 percent of their total marketing budgets to 15.8 percent over the next five years.

The bright spot in the survey is that CMOs report their highest optimism for the U.S. economy in four years. On a scale of 0 to 100 with 100 being most optimistic, CMOs scored the economy at 65.7, up nearly 20 points compared to August 2009. Nearly 50 percent of CMOs said they were “more optimistic” about the overall U.S. economy compared to last quarter.

That optimism suggests a rebound in consumer spending, which would translate into higher sales figures for companies. But attributing higher revenue to social media might be a tough case to make if it’s hard to tell whether the better results come from social media or an improving overall economy.

So are the marketing departments just throwing good money after bad? We’ll see. The next time you buy something based on a Tweet or a Facebook post, let me know.