First Google, then Facebook, then...who?

In the battle for digital ad dollars, it's Google (GOOGL), 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 (FB) 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. (Tweet This) Way below them, the No. 3, 4 and 6 players—Microsoft (MSFT), Yahoo and AOL—are losing market share, and nobody below them claims even 2 percent. Twitter ranks fifth at 2.3 percent.

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With U.S. online advertising forecast to grow 16 percent this year to $58.6 billion and reach almost $75 billion in 2017, and with Google and Facebook constantly adding services on the Web and smartphones, ad departments are looking for ways to diversify their spending.

They're testing ads on Twitter (TWTR), LinkedIn, Yelp (YELP) and Pandora as well as services such as Pinterest and Snapchat. But none in the U.S. has emerged as a clear challenger to the Google-Facebook duopoly, a concern for brands that worry about losing visibility when the top two companies tweak an algorithm or raise prices.

Brands "don't want all their eggs in one basket," said Cathy Boyle, an analyst at eMarketer in New York. "They really do want a third player, but at this point we don't have one with the combined data and reach."

Google and Facebook are ubiquitous. Their users are scouring the web while logged in, enabling the tracking of every click and purchase and producing the kind of targeting capabilities that advertisers so covet.

Facebook is a distant second to Google, but growing much faster. The Menlo Park, California-based company is scheduled to report first-quarter results on Wednesday, and analysts expect sales growth of 42 percent to $3.56 billion and earnings per share of 40 cents up from 34 cents a year earlier, according to a Thomson Reuters survey.

Google, based in Mountain View, California, follows on Thursday. Expectations are for gross sales of $17.5 billion, an increase of 14 percent from a year earlier, with EPS rising to $6.60 from $6.27.

EMarketer sees Google's U.S. market share dropping from 36.6 percent this year to 32.5 percent in 2017, with Facebook gaining almost 2 percentage points to 13.4 percent. So the combined market share, while still dominant, is slowly slipping.

And with the European Union this week filing antitrust charges against Google for allegedly favoring its own properties over rivals in shopping searches, the distraction may open doors for others to enter.

Who's best positioned to take advantage?

Twitter, Amazon.com (AMZN), LinkedIn and Yelp are all in the top 10 in the U.S., with market shares of between 0.8 percent and 2.3 percent and rising.

Read More EU files antitrust charges against Google

Patrick Llewellyn, chief executive officer of 99designs, is struggling in his search to find marketing channels that can actually attract new users. The company, which connects web designers with relevant businesses, spends about 70 percent of its marketing budget on Google, 10 percent on Facebook and spreads the rest across an assortment of sites and services.

Though Google is the most effective source for 99designs, it's getting expensive. Keywords such as "logo design" and "website design" have doubled in price in the past 12 to 18 months on Google's search engine, Llewellyn said, making what was once a relatively cheap marketing channel quite pricey. Facebook remains the best option for retargeting, or sending ads to people who have already expressed some level of interest in the brand, he said.