• by Mark Walsh  @markfwal, April 27, 2012

In a report last September, Yankee Group dubbed Apple, Google, Amazon and Facebook the “Four Horsemen” of what it called the mobile content revolution. The mobile-focused research firm has since updated its findings, estimating that more than half the collective $200 billion in revenue of the four tech giants came from mobile sources.

Considering that Apple’s portion accounted for the vast majority of that $100 billion-plus in mobile revenue, however, Gladys Knight and the Pips might be a more apt metaphor. Still, the companies together drove three-quarters (76%) of worldwide mobile app downloads and ad revenue last year.

The big payoff is yet to come, with mobile content revenue expected to grow at an annual rate of 35% in the next three years, from $10 billion in 2011 to more than $43 billion in 2015. That compares to only 10% annual growth for mobile device sales.

When it comes to determining mobile revenues, the report authored by Yankee Group analyst Carl Howe concedes that Amazon, Apple, Facebook and Google do not consistently report how much they make from mobile products and services. By combining public data with its own models, however, the firm believes it has come up with reasonable estimates.

Apple’s mobile-related sales clearly dwarf those of the other three. Combined revenue from 2011 sales of the iPhone, iPad and iPods was $92.2 billion. In addition, its music, video and app stores sold $6.9 billion worth of content, of which Yankee Group estimates $5.4 billion was for mobile devices. That brings the total to just under $100 billion, amounting to more than three-quarters of Apple sales.

Google mobile revenue has come mainly through search, which company CEO Larry Page pegged at $2.5 billion last year. Another estimated $1 billion in downloaded apps and content from Google Play brings the 2011 total to $3.5 billion, or 7% of overall revenue. The report faults Google’s reliance on advertising as an inefficient way to capitalize on mobile content compared to direct sales.

Like Apple, Amazon has both device and mobile content sales, although on a much smaller scale to date. Yankee figures the company sold roughly $1.8 billion worth of Kindle readers and tablets last year, along with $4.3 billion of other manufacturers’ mobile devices. With $2 billion more from sales of e-books, music and other digital media, mobile revenue comes to $8.1 billion -- equal to 17% of total sales.

Of the so-called Four Horsemen of mobile, Facebook stands out as the straggler. While boasting half a billion monthly active mobile users globally, the company reiterated in its latest securities filing that it doesn’t yet generate any meaningful revenue from its mobile products. Until last month, when it began including Sponsored Stories ads in mobile users’ news feeds, Facebook has not shown ads in its mobile apps or its mobile site.

A recent forecast from Pivotal Research Group senior analyst Brian Wieser projects Facebook’s mobile ad revenue will surge from nothing to nearly $500 million in the U.S. by 2017. That does not include any revenue it could also add by monetizing digital media payments and e-commerce coming through its platform on mobile devices. Still, that amount would only be a small portion of Facebook’s overall projected ad revenue of $12.6 billion in five years.

If Facebook wants to maintain its $100 billion valuation, Howe said the social networking powerhouse must ramp up its mobile monetization. He suggests that the company should  introduce premium Android and iOS apps, which even at $2.99 apiece would translate into more than a $500 million business if sold to half Facebook’s mobile subscribers.

Still, the study asserts that Amazon, Apple, Facebook and Google will benefit most from the rapidly growing mobile content business in the coming years as the dominant players in the space. Of the projected $43 billion total in three years, about $30 billion would come from advertising and $13.5 billion from mobile app store revenue.