When it comes to digital advertising, there are really just two alternatives -- and then the "others." Despite concerns surrounding Google's (NASDAQ:GOOG) (NASDAQ:GOOGL) mobile ad results, it's far and away the leader of the mobile digital ad pack. Not surprisingly, Google is mum on how much of its $17.73 billion in sales last quarter were the result of its renewed emphasis on mobile ads -- particularly search -- but based on some new data, it's clear it remains head and shoulders above the rest.

Though not quite in the same ballpark as Google, Facebook (NASDAQ:FB) has quickly become the search king's biggest rival for the ad dollars of digital marketers. For the first time ever, Facebook cracked the $4 billion revenue barrier last quarter. Even more impressively, a whopping 76% of Facebook's $3.83 billion in ad sales in Q2 were of the mobile variety.

According to recent estimates, winning the mobile ad wars isn't just a nicety, it's quickly becoming a necessity. And if the current trend continues, it appears Google may be heading in the wrong direction.

Emarketer Total Digital Ad Spend

Just the facts
How big is big? As per eMarketer, mobile digital advertising will jump to more than $30 billion this year: that's up from $19.15 billion in 2014. Incredibly, digital ad spend, in total, will outpace TV sales in just two years. That trend is expected to continue through 2019, when more than 40% of marketing budgets will be directed toward digital media, dwarfing television's 34.6%.

Mobile digital spend will outpace desktop ads in 2015 thanks to a 59% increase on spots directed to consumers on the go. In four years, nearly 70% of digital marketing dollars will be directed to mobile ads. That equates to a ton of upside for sites like Google and Facebook, which both boast a strong mobile presence.

Emarketer Mobile Ad Spend

As it stands today, Google is the hands-down leader in the U.S. when it comes to mobile advertising, commanding nearly a third of each mobile digital advertising dollar. Facebook is hardly an afterthought, though; its expected 19.4% mobile ad market share this year may not impress stacked up against Google. That said, the seemingly wide disparity between the two digital ad behemoths warrants further investigation given that the two online ad leaders appear to be heading in opposite directions.

Score one for Facebook
Despite their obvious difference in size based on market capitalization and revenue, Facebook has something going for it that Google should take note of: Its piece of the advertising pie -- mobile and desktop -- continues to grow. Not coincidentally, Google's ad market share, while it remains well above Facebook's, is slowly, but surely, slipping, and that trend is expected to continue.

For example, that 32.9% U.S. mobile ad share that Google will garner this year? That's down significantly from 2014's 36.9%. Facebook's piece of the mobile ad market was a "mere" 18.5% last year compared to this year's 19.4%. The same trend -- Google losing market share to Facebook -- applies across all digital media outlets in the U.S., too.

In 2014, Google owned 41.6% of all digital ad spending in the U.S., while Facebook eked out a respectable, if not outstanding, 10.6%. In just two years, Google is expected to lose about six percentage points of market share, and Facebook will gain those same six percentage points of U.S. digital ad dollars, growing to 16.1% of the overall market.

By no means is Google in dire need of radical change to stem the digital ad spending tide. Any company that generates $17.7 billion in quarterly revenue, and continues to grow sales in the double digits year over year as Google consistently does, is a sound long-term growth opportunity. However, as the recent data demonstrates, Facebook appears to have even more upside as the world goes mobile.

Based on their respective stock prices these last few, turbulent weeks, it seems investors recognize that, of the two digital ad leaders, Google needs to keep its eyes on Facebook. Despite the market's ups and downs, Facebook is inching its way toward the vaunted $100 a share plateau -- no easy feat in today's economic environment.

Should Google worry about the digital ad trend, in general, and mobile, in particular, swinging in Facebook's favor? You bet it should.

Tim Brugger has no position in any stocks mentioned. The Motley Fool owns and recommends Facebook, Google (A shares), and Google (C shares). Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.