What do you get for your digital-marketing dollars? Figuring out how much revenue or brand value companies are generating from their investments in digital media seems to be getting more and more complex. While it's simple to measure click-through rates on the Web, impressions on mobile devices, tweets on Twitter, and "likes" on Facebook, the business impact in the real world is notoriously difficult to quantify. So as marketers "spread their spending over a widening set of digital options," says Chuck Richard, vice president of Outsell, a research firm based in Burlingame, California, "they need more accountability."

Accountability becomes even more urgent when you consider the numbers. In 2010, for the first time ever, spending on digital media will surpass spending on print media in the United States, Outsell forecasts. The $120 billion that companies are shelling out this year includes all digital advertising and marketing efforts, including website development, edging past the $111.5 billion to be invested in print-based marketing. Meanwhile, the U.K. has become the first major economy in which spending on Internet advertising has overtaken spending on television advertising. These are milestones in a movement that shows no sign of abating, as digital continues to claim an ever larger share of the corporate marketing budget.