Digital ad revenue in the U.S. grew by more than 20% last year to a record $72.5 billion, according to the Interactive Advertising Bureau. That's the good news.

The bad news—at least for those who dislike duopolies—is that some estimates by other industry experts show that virtually all of growth in digital ad spending went to Google and Facebook, which already account for more than three-quarters of the U.S. digital ad market.