Google & Facebook account for 49'% of US digital ad spend & 52% of mobile ad spend

"In the battle for digital ad dollars, it's Google, Facebook and then who?
That's the question marketers are asking as they try to spend effectively on the Web, and even more so on mobile, where traffic is shifting.
Heading into quarterly earnings reports next week, Google and Facebook utterly dominate digital advertising, according to eMarketer.
The two alone control 49 percent of the U.S. digital advertising market and 52 percent of mobile ads.  Way below them, the No. 3, 4 and 6 players—Microsoft, Yahoo and AOL—are losing market share, and nobody below them claims even 2 percent. Twitter ranks fifth at 2.3 percent."

Source:  CNBC, 17th April 2015