eMarketer say that it formulates the estimates by analyzing data from a variety of research firms, government agencies, media outlets and company reports.
Yahoo posted quarterly earnings and revenue that failed to match analysts' estimates on Tuesday evening but shares gained in afterhours trading after it said it wouldn't be selling as much of its stake in Chinese online giant Alibaba. Shares, however turned around and were trading lower by 1.8 percent at 10:00 a.m. London time on Wednesday ahead of the U.S. open.
CEO Marissa Mayer addressed concerns over the company's turnaround in Yahoo's earnings conference call , shortly after Tuesday's results.
Read MoreYahoosales outlook weak; shares wobble on Alibaba
"Given our top priority of long-term sustainable growth, we not satisfied," she said. "In the case of Yahoo, I have stated in the past that we believe a transformation of this size and scale will take multiple years. And we continue to believe that is the case today."
Meanwhile, Google is set to comfortably keep its title as the king of online advertising, according to eMarketer. It said it would dominate the global digital ad market this year, netting 31.45 percent of the digital dollars invested by advertisers worldwide. This comes despite it losing a little market share, which has been put down to a sign of its "maturing" ad business in comparison to competitors.